You guilds you can easily(a relative term)join are the Shao-Lin, Crusaders, Taniel clerics, Asral clerics and the Rangers.
Thieves are bit secretive and yes, hard to play so they can prove to be very hard to join.
There are druids but I think there are not many druid players active currently. I am not sure. These guys are mostly pacifists so if you like to beat stuff up, this is not your place.
The evil guys are the Sathonys clerics and the Order. Playing an evil character is bit hard here, so you might be better off starting you as a neutral/goodish until you get your bearings.
There is a mage guild but the requirements for joining mean you have to have an experienced character to join. You can have minor magics or miracles depending on your choices however even if you are not a guild mage.
Humans and the city of Arborea claim the most land, more or less and are affiliated with the Asrals. All three layman guilds, skalds, scribes and alchemists can be found from their lands. Often at odds with Elvandar, the city of elves, for reasons historical and philosophical alike.
Elvandar was mentioned, and is affiliated with Crusaders, Rangers and Taniel clerics. Lots of moral superiority to be found there
(With affiliated I mean you are most likely to run into these guys there, and usually the guys from the big guild hold some power/influence there as well)
You should not be hasty about joining a guild. You will not miss out much and you will have more freedom of choice. You can learn lots of skills and stuff without ever joining an occupational guild. I suggest you talk to people IC.
There are vampires and werewolves in Geas but both are rather rare. Chances are it will take a long while until you run into one.