Our co-op was a really friendly place, with a mix of outgoing and more introverted parents. For me, as an introvert, it was a great way to meet a bunch of people because we had shared interests--our kids and the co-op itself, for which we shared responsibility.
That said, I'm sure that the friendliness varies by co-op, and even by year, so it's worth trying to find out about the particular co-op and meet some current parents.