Space Sync for Confluence supports mention users in external sites.
To enable it, you, who is a space administrator, explicitly allow the destination site to mention users in your site.
If you allow the destination space to mention users in your site, every user who can view the destination space can search users in your site. Thus, users in the destination site can get all users on your site. User data includes only user id and names. Others like E-mail addresses and avatars are not included.
The operations are executed in the Add-ons tab in the Space setting screen.
Open edit screen for the setting you want to change whether mention from the external site is enabled.
Change toggle under “Allow mention from the destination” label.
This toggle is only shown for established sync with an external site. You have to establish sync before changing allowing mention.