Some services, notably user directories and gateways to legacy IM systems, allow searching. Searching in Jabber generally means searching for someone's JID, but the protocol is general enough to support most databases.
To search a service, either type M-x jabber-get-search or select it from the Service menu, which is opened by typing C-c C-s.
Just like with registration, this command sends a request for a search form, and displays it if and when the response arrives. Enter your search and submit it. Search results will be displayed in a different browse buffer.