I'm having trouble purchasing spells in MM2. It says that I haven't joined the guild yet. I'm sure I'm missing something but I am not sure what it is. Can someone help?

You are in Middlegate, I suppose.
I attached a map of the town: you have to go to location 6 in the northwest part of the town. There you will find a "sleepy conjurer" who can enroll you in the local mages' guild for 20 gold pieces.

similarly the other towns will have the same deal of a person elsewhere in town to sell membeships for that towns mage guild

Thank you guys. I thought I searched every spot and wondered what I was missing. Obviously I missed this room. Thanks again.