Each player must have their Punkbuster GUID listed on their teams roster page to be considered eligible for a match.

To get your Punkbuster GUID

Open UO, join a server (one with Punkbuster running), hit the "~" key and input /pb_plist in your console. Take note of the 8 digits displayed next to your name. This is your PBGUID. This is all you will need to input next to your name.

Another way is to load up the game, pull down the console, and type /pb_myguid. Note the last 8 characters.