When you add users in Pidgin using Buddies -> Add Buddy make sure you chose the account of the user who is sending the request and specify the other user's user name as a Jabber ID: e.g. john@domain.com
The other user should receive an authorization request. XMPP clients can have very different user interfaces for this kind of stuff, and most of them are confusing in my opinion. (The service for which I use Openfire uses a custom XMPP chat client.)