Server to server has some security attached to it in the administration console. If you look in the administration console of your own server, you’'ll see that you can configure the server to enable/disable the feature, select who is allowed to connect to your server (anyone, or white list), and what servers are not allowed to connect (this only applies if ‘‘allowed to connect’’ is ‘‘anyone’’).
If you are using a white list, you must add the other server to your allowed list. If your server is set to allow anyone, then all you need to do is ensure you have the service enabled and the remote server will allow your server to connect to it.
The server on the other end has the same type of options - if they’‘re using a white list, then they need to add your server to it’‘s list. If it’'s a public server, it may allow any server to connect (though not necessarily - ‘‘public’’ may refer to who can connect with a client, not necessarily with a server).
Unless you plan to let any server connect to yours, I recommend using a white list. For my money, I don’'t like to allow just anyone out there to connect to a machine I administer - especially in a server to server type capacity.
As for the sharing of users, that’‘s not exacly how server to server works. Your server has it’‘s own local users, and the other server(s) have their own local users. When a user on one server wants to communicate with a user on another server, the initiating server will try to establish a connection to the remote server (if it’‘s not already established) and deliver the message. You won’‘t see a list of the other server’'s users on your server anywhere (unless you create a group and add remote users to the group by hand).
Clients, however, can add remote users to their rosters (their list of users they talk to), and your server will reach out to the remote servers when a user requests communication with the remote user or the local user signs on and presence requests go out to remote servers so online/offline/away status can be retrieved.
I hope that helps.