Individual XMPP domains are stand-alone. There’s no knowledge about each-others users.
Maybe you can utilize subscription requests? If one user adds another user on its roster, typically, a ‘presence subscription request’ is send out. The other end could use this to “add the user back”.
Other than that, you might need to resort to a shared directory of sorts (LDAP/AD?) but you’d still need to figure out a way to add the correct (remote) users to the roster of the local users.
I’m not seeing how you can do this with standard functionality. You might need to create custom code for this.
I still fail to see exactly what the use case is. That might limit me in finding a suitable solution.