Currently there is no (easy) way for a remote client to discover a list of all user in a Jira installation. The best that you can do (if you are an admin user) is call getUsers(Group) where group is the jira-users group. It would be really handy if there were a service to get a list of all users in the system. With this information it would be possible for remote clients to provide user lookup facilities and mappings between user id's and real names.
However, I do understand that adding such a service opens the system up to abuse. It would be trivial to write a client that could harvest a list of valid email addresses from the system. Requiring the user to login before calling the service may help this a bit.
Here is the RPC service code: