NodeBB/NodeBB — pull request #1829 by akhoury (2014-07-15 17:40:49+00:00)
Title: Group.join() callback made optional

because [User.create()](https://github.com/NodeBB/NodeBB/blob/master/src/user/create.js#L162) uses it without a callback, and it was barfing if there was an error.