You can only be a member of one co-op at a time. If you are already in one, you will have to leave it before you can join another one. To leave a co-op, tap the Co-op Building and then the Leave button. Once you’ve done so, you’ll be free to join another co-op or create your own!
Bear in mind that if you switch co-ops too often, you'll have to wait for a while before you can join a new one.