In the user dashboard, the follow user field can be used for following users of the current instance, but also for following users of external instances supporting the ActivityPub and Webfinger protocols.

To do so, the input must be in the form @username@servername.

Once a follow request sent, the user will eventually appear in the list of followed users of the user dashboard.

The followed user will only appear in the following list if the following request is accepted. This is usually done automatically but can be subject to manual user validation according to the settings of the external instance.

Similarly, external user that support Webfinger can follow users of an XWiki instance by using their Webfinger identifiers.
Finally, external users can send messages to XWiki users. The received message appears in the notifications of the user.