How do I join a Fellowship?

To join an existing fellowship you must have a Fellowship Registration Insignia. You can get one by talking to Randall of the Fellows (ask him "how"). You must be in the Plane of Knowledge and the leader of the fellowship you wish to join must be near Randall in order to invite you. If you already belong to another fellowship you will have to leave that one first. (You can leave a fellowship from anywhere in the EverQuest world.)