The United Pirates have a village in Trammel, close to the shrine of Honor. We aren't there a whole lot, as we prefer to travel a lot. But if you happen to find us there, we're happy to play with you.
Our page:
http://www.aschulze.net/ultima/pirates/
Our maps:
http://www.aschulze.net/ultima/pirates/maps.htm
More resources by our Captain:
http://www.aschulze.net/ultima/guide/index.htm
Be sure to check out the large maps, they're really quite excellent
Alternatively, there is Olympus, a mainly human city (guild name Purple Guardians of Honor or PGoH) which lies on the western edge of the swamps north of Trinsic (also Trammel). I don't have a map to there and their web page is in dire need of updating, but it can get quite lively there if you visit at the right time. We do have a rune to Olympus in the pirate rune-library, which is included in the map section of the web page (Runes in the lower right rune book).