When following or messaging users, multiple solution exist to identify the users.
If the user is an XWiki account, the address of her user page can be used, for instance http://xwiki_domain/xwiki/bin/view/XWiki/UserName.
If the user is an XWiki account of the current instance, his user name can be used directly, for instance UserName.
Finally, if the user is an account of a server supporting Webfinger, his webfinger identifier can be used, for instance @UserName@nextcloud_domain.com.
