It does have the connotation of someone who gets wasted and parties while attending these festivals, but the overall meaning is based on simply frequenting festivals.
"So you said Steve might be coming to forest this year?"
"Yeah, but I don't think it's gonna happen, he's not much of a festie."
Those who frenquent renfaires as customers, often with seasons passes, but who are not part of the performing cast or otherwise work for the renfaire (ie. rennies). Usually some sort of badges are worn by workers and performers of a renfair as proof that they are employees. Because they add to the general atmosphere of a renfaire, most renfaires encourage festies to wear garb, thus creating the need for the badges to tell worker from patron.