diff --git a/app/client/aclui/AccessRules.ts b/app/client/aclui/AccessRules.ts
index e3949f65..8bed5276 100644
--- a/app/client/aclui/AccessRules.ts
+++ b/app/client/aclui/AccessRules.ts
@@ -35,7 +35,7 @@ import {
   UserAttributeRule
 } from 'app/common/GranularAccessClause';
 import {isHiddenCol} from 'app/common/gristTypes';
-import {isObject} from 'app/common/gutil';
+import {isNonNullish} from 'app/common/gutil';
 import {SchemaTypes} from 'app/common/schema';
 import {MetaRowRecord} from 'app/common/TableData';
 import {
@@ -1331,7 +1331,7 @@ function syncRecords(tableData: TableData, newRecords: RowRecord[],
     const newRec = newRecordMap.get(uniqueId(r));
     const updated = newRec && {...r, ...newRec, id: r.id};
     return updated && !isEqual(updated, r) ? [r, updated] : null;
-  }).filter(isObject);
+  }).filter(isNonNullish);
 
   console.log("syncRecords: removing [%s], adding [%s], updating [%s]",
     removedRecords.map(uniqueId).join(", "),
diff --git a/app/common/gutil.ts b/app/common/gutil.ts
index 1887eb50..5ca6af0d 100644
--- a/app/common/gutil.ts
+++ b/app/common/gutil.ts
@@ -899,9 +899,9 @@ export function isAffirmative(parameter: any): boolean {
  * Returns whether a value is neither null nor undefined, with a type guard for the return type.
  *
  * This is particularly useful for filtering, e.g. if `array` includes values of type
- * T|null|undefined, then TypeScript can tell that `array.filter(isObject)` has the type T[].
+ * T|null|undefined, then TypeScript can tell that `array.filter(isNonNullish)` has the type T[].
  */
-export function isObject<T>(value: T | null | undefined): value is T {
+export function isNonNullish<T>(value: T | null | undefined): value is T {
   return value !== null && value !== undefined;
 }
 
diff --git a/app/common/parseDate.ts b/app/common/parseDate.ts
index 0d7d98db..a98b29d3 100644
--- a/app/common/parseDate.ts
+++ b/app/common/parseDate.ts
@@ -1,7 +1,7 @@
 import escapeRegExp = require('lodash/escapeRegExp');
 import last = require('lodash/last');
 import memoize = require('lodash/memoize');
-import {getDistinctValues, isObject} from 'app/common/gutil';
+import {getDistinctValues, isNonNullish} from 'app/common/gutil';
 // Simply importing 'moment-guess' inconsistently imports bundle.js or bundle.esm.js depending on environment
 import * as guessFormat from '@gristlabs/moment-guess/dist/bundle.js';
 import * as moment from 'moment-timezone';
@@ -346,7 +346,7 @@ export function guessDateFormat(values: Array<string | null>, timezone: string =
  * May return null if there are no matching formats or choosing one is too expensive.
  */
 export function guessDateFormats(values: Array<string | null>, timezone: string = 'UTC'): string[] | null {
-  const dateStrings: string[] = values.filter(isObject);
+  const dateStrings: string[] = values.filter(isNonNullish);
   const sample = getDistinctValues(dateStrings, 100);
   const formats: Record<string, number> = {};
   for (const dateString of sample) {
diff --git a/app/server/lib/FlexServer.ts b/app/server/lib/FlexServer.ts
index 1d8370fb..9d9c1cfb 100644
--- a/app/server/lib/FlexServer.ts
+++ b/app/server/lib/FlexServer.ts
@@ -6,7 +6,6 @@ import {encodeUrl, getSlugIfNeeded, GristLoadConfig, IGristUrlState, isOrgInPath
 import {getOrgUrlInfo} from 'app/common/gristUrls';
 import {UserProfile} from 'app/common/LoginSessionAPI';
 import {tbind} from 'app/common/tbind';
-import {UserConfig} from 'app/common/UserConfig';
 import * as version from 'app/common/version';
 import {ApiServer} from 'app/gen-server/ApiServer';
 import {Document} from "app/gen-server/entity/Document";
@@ -103,7 +102,7 @@ export class FlexServer implements GristServer {
   public housekeeper: Housekeeper;
   public server: http.Server;
   public httpsServer?: https.Server;
-  public settings: any;
+  public settings?: Readonly<Record<string, unknown>>;
   public worker: DocWorkerInfo;
   public electronServerMethods: ElectronServerMethods;
   public readonly docsRoot: string;
@@ -802,22 +801,24 @@ export class FlexServer implements GristServer {
     });
   }
 
-  // Load user config file from standard location.  Alternatively, a config object
-  // can be supplied, in which case no file is needed.  The notion of a user config
-  // file doesn't mean much in hosted grist, so it is convenient to be able to skip it.
-  public async loadConfig(settings?: UserConfig) {
+  /**
+   * Load user config file from standard location (if present).
+   *
+   * Note that the user config file doesn't do anything today, but may be useful in
+   * the future for configuring things that don't fit well into environment variables.
+   *
+   * TODO: Revisit this, and update `GristServer.settings` type to match the expected shape
+   * of config.json. (ts-interface-checker could be useful here for runtime validation.)
+   */
+  public async loadConfig() {
     if (this._check('config')) { return; }
-    if (!settings) {
-      const settingsPath = path.join(this.instanceRoot, 'config.json');
-      if (await fse.pathExists(settingsPath)) {
-        log.info(`Loading config from ${settingsPath}`);
-        this.settings = JSON.parse(await fse.readFile(settingsPath, 'utf8'));
-      } else {
-        log.info(`Loading empty config because ${settingsPath} missing`);
-        this.settings = {};
-      }
+    const settingsPath = path.join(this.instanceRoot, 'config.json');
+    if (await fse.pathExists(settingsPath)) {
+      log.info(`Loading config from ${settingsPath}`);
+      this.settings = JSON.parse(await fse.readFile(settingsPath, 'utf8'));
     } else {
-      this.settings = settings;
+      log.info(`Loading empty config because ${settingsPath} missing`);
+      this.settings = {};
     }
 
     // TODO: We could include a third mock provider of login/logout URLs for better tests. Or we
diff --git a/app/server/lib/GranularAccess.ts b/app/server/lib/GranularAccess.ts
index 3c8709b0..29624e60 100644
--- a/app/server/lib/GranularAccess.ts
+++ b/app/server/lib/GranularAccess.ts
@@ -16,7 +16,7 @@ import { ErrorWithCode } from 'app/common/ErrorWithCode';
 import { AclMatchInput, InfoEditor, InfoView } from 'app/common/GranularAccessClause';
 import { UserInfo } from 'app/common/GranularAccessClause';
 import { isCensored } from 'app/common/gristTypes';
-import { getSetMapValue, isObject, pruneArray } from 'app/common/gutil';
+import { getSetMapValue, isNonNullish, pruneArray } from 'app/common/gutil';
 import { canEdit, canView, isValidRole, Role } from 'app/common/roles';
 import { FullUser, UserAccessData } from 'app/common/UserAPI';
 import { HomeDBManager } from 'app/gen-server/lib/HomeDBManager';
@@ -1039,7 +1039,7 @@ export class GranularAccess implements GranularAccessForBundle {
       this._makeAdditions(rowsAfter, forceAdds),
       this._removeRows(action, removals),
       this._makeRemovals(rowsAfter, forceRemoves),
-    ].filter(isObject);
+    ].filter(isNonNullish);
 
     // Check whether there are column rules for this table, and if so whether they are row
     // dependent.  If so, we may need to update visibility of cells not mentioned in the
diff --git a/app/server/lib/GristServer.ts b/app/server/lib/GristServer.ts
index 92f66cbe..04e0e113 100644
--- a/app/server/lib/GristServer.ts
+++ b/app/server/lib/GristServer.ts
@@ -22,6 +22,7 @@ import * as express from 'express';
  */
 export interface GristServer {
   readonly create: ICreate;
+  settings?: Readonly<Record<string, unknown>>;
   getHost(): string;
   getHomeUrl(req: express.Request, relPath?: string): string;
   getHomeUrlByDocId(docId: string, relPath?: string): Promise<string>;
diff --git a/app/server/lib/ICreate.ts b/app/server/lib/ICreate.ts
index ed4a6650..7b36ef49 100644
--- a/app/server/lib/ICreate.ts
+++ b/app/server/lib/ICreate.ts
@@ -46,10 +46,15 @@ export interface ICreateStorageOptions {
   create(purpose: 'doc'|'meta', extraPrefix: string): ExternalStorage|undefined;
 }
 
+export interface ICreateNotifierOptions {
+  create(dbManager: HomeDBManager, gristConfig: GristServer): INotifier|undefined;
+}
+
 export function makeSimpleCreator(opts: {
   sessionSecret?: string,
   storage?: ICreateStorageOptions[],
   activationMiddleware?: (db: HomeDBManager, app: express.Express) => Promise<void>,
+  notifier?: ICreateNotifierOptions,
 }): ICreate {
   return {
     Billing(db) {
@@ -63,8 +68,9 @@ export function makeSimpleCreator(opts: {
         }
       };
     },
-    Notifier() {
-      return {
+    Notifier(dbManager, gristConfig) {
+      const {notifier} = opts;
+      return notifier?.create(dbManager, gristConfig) ?? {
         get testPending() { return false; },
         deleteUser()      { throw new Error('deleteUser unavailable'); },
       };
diff --git a/app/server/lib/idUtils.ts b/app/server/lib/idUtils.ts
index 23227f37..9c57430e 100644
--- a/app/server/lib/idUtils.ts
+++ b/app/server/lib/idUtils.ts
@@ -40,3 +40,10 @@ export function makeForkIds(options: { userId: number|null, isAnonymous: boolean
 export function getAssignmentId(docWorkerMap: IDocWorkerMap, docId: string): string {
   return docId;
 }
+
+// Get the externalId to use for an AppSumo user.  AppSumo identifies users by
+// an activation email, so we just use that (with an appsumo/ prefix it allow
+// for other families of id in the future).
+export function getExternalIdForAppSumoUser(email: string) {
+  return `appsumo/${email}`;
+}
diff --git a/test/server/docTools.ts b/test/server/docTools.ts
index 32d7be52..aa272222 100644
--- a/test/server/docTools.ts
+++ b/test/server/docTools.ts
@@ -154,6 +154,7 @@ export async function createDocManager(
 export function createDummyGristServer(): GristServer {
   return {
     create,
+    settings: {},
     getHost() { return 'localhost:4242'; },
     getHomeUrl() { return 'http://localhost:4242'; },
     getHomeUrlByDocId() { return Promise.resolve('http://localhost:4242'); },
diff --git a/test/server/lib/Authorizer.ts b/test/server/lib/Authorizer.ts
index 6c4af796..8b90a1bf 100644
--- a/test/server/lib/Authorizer.ts
+++ b/test/server/lib/Authorizer.ts
@@ -22,7 +22,7 @@ async function activateServer(home: FlexServer, docManager: DocManager) {
   home.addDocWorkerMap();
   home.addAccessMiddleware();
   dbManager = home.getHomeDBManager();
-  await home.loadConfig({});
+  await home.loadConfig();
   home.addSessions();
   home.addHealthCheck();
   docManager.testSetHomeDbManager(dbManager);