there were not any guilds in Ultima Underworld, nor could you be anything but human, i dont think.
the height / race / classless guild joining stuff sounds like a TES game, Arena / Daggerfall / Morrowind. I never played Battlespire, maybe it was that?
But it might be a hard hunt. there were fucktons of games like this, but I'll try and drop a few names for you to look up. Lands of Lore (series). Wizardry (series). Eye of the Beholder (series). Gold Box Games (different TSR games using a similar game engine). Might and Magic (series, not to be confused with HoMM).
I'm sure there are more, and some of those probably fail some of your criteria. I can think of others, but all the others that are coming to mind explicitly dont match something you described. The multiplayer thing is sort of throwing me off, though. I know its a feature that lots of games had, but that I never payed much attention to.
Was it internet multiplayer? direct modem dial? LAN based? did you have to hook up a serial or parallel cabel between your computers? Was this on an Atari ST using MIDI cables?
Did you just whistle into a phone and your friend would whistle back based on what his character was doing?
i wish i could say for sure some of these things, i will try:
you were not classless. you gained exp for your specific class that you were but you could change your class at the hub town (in the guild hall). the hub town was diffrent from the main game, it wasent first person but more like the hub town in shining in the darkness.
as for the multiplayer, it was weird, i think it was lan/ip based, you could see the other player(s) in the game but you couldent interact with them except for tradeing (maybe we just didnt know how to...). i think it had a desync problem but i cant recall.
i will try looking up those series.
this is really bugging me arrgh it could even be on my dead laptop as we speak but its dead >.<
i do remember seeing it in the store which prompted me to look into it online. i've gone through home of the under dogs with no luck.
is there a list online i could look through?
-edit: i do remember that there was old style 3d models for the enemies, like polygons with textures. but thats it for tonight, my wife is calling me to the bed.. this will haunt me to my grave though x.x