An easy way to bond a community together is to send them to war.
Not so much the gory guns and killing kind, but a war of pride. If your online community has a big rival community, challenge them to a contest. It can be a quiz, a content creation contest, a battle of skills – just keep it relevant to your community.
Perhaps find some respected independent judges to rate who wins. This should rally your community together, and attract members from your rival.
Have fun with it. The winner gets to brag, the loser demands a rematch.