ActivityTimeline automatically synchronizes user list updates from Jira during the daily background synchronization, which usually runs at night. This includes newly added users who belong to the Jira groups configured for synchronization.
If you need to update the user list sooner, you can run the synchronization manually:
Configuration → Synchronization → Refresh Users → Refresh Users
After the synchronization is completed, you will be redirected to the Users page, where the updated list of users will be displayed.
Please note by default, ActivityTimeline synchronizes users from the Jira groups mentioned below:
jira-software-users
jira-core-users
jira-users
jira-software-users-<INSTANCE_NAME>
Users deleted in Jira will remain in ActivityTimeline until they are manually removed from Configuration → Users → Manage User → 
If some users are missing
First, check whether the relevant Jira groups exist and whether the required users are included in those groups.
If users were recently added to a Jira group, go to:
Configuration → Users → Groups → Refresh Groups
Then run Refresh Users again from:
Configuration → Synchronization → Refresh Users
Alternatively, if you have another Jira group that contains all required users, add it to the Extra groups to search users in field in the user synchronization settings, then click Refresh Users.
Additional settings
You can also configure how often ActivityTimeline should synchronize new users from Jira and which default role should be assigned to them after synchronization. The automatic user synchronization can be set to run daily, weekly, or never if you do not want new users to be added automatically.
