diff --git a/lib/utils/conversion-utils.js b/lib/utils/conversion-utils.js
--- a/lib/utils/conversion-utils.js
+++ b/lib/utils/conversion-utils.js
@@ -5,7 +5,7 @@
 import type { TInterface, TType } from 'tcomb';
 
 import { convertIDToNewSchema } from './migration-utils.js';
-import { assertWithValidator, tID } from './validation-utils.js';
+import { assertWithValidator, tID, tUserID } from './validation-utils.js';
 import {
   getPendingThreadID,
   parsePendingThreadID,
@@ -57,6 +57,21 @@
   return convertObject(outputValidator, data, [tID], conversionFunction);
 }
 
+function extractUserIDsFromPayload<T>(
+  outputValidator: TType<T>,
+  data: T,
+): $ReadOnlyArray<string> {
+  const result = new Set<string>();
+  const conversionFunction = (id: string) => {
+    result.add(id);
+    return id;
+  };
+
+  convertObject(outputValidator, data, [tUserID], conversionFunction);
+
+  return [...result];
+}
+
 function convertObject<T, I>(
   validator: TType<I>,
   input: I,
@@ -141,5 +156,6 @@
 export {
   convertClientIDsToServerIDs,
   convertServerIDsToClientIDs,
+  extractUserIDsFromPayload,
   convertObject,
 };