There's no page to access other users' XMPP/Jabber info. However, the construction of it is pretty straightforward: <group_id>_<user_id>@chat.hipchat.com. You can either ask those users to go to the /account/xmpp page and copy and paste their JID to you, or you can pull their IDs by going to hipchat.com/admin/users, clicking on the user you want, and using the ID found in the url (should appear something like hipchat.com/admin/user/USER_ID)
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