Alliances are a way to help increase your rank within the game. A new player does not appear to be able to join an alliance until after a set amount of time (or possibly a number of games) has elapsed. There is also a Forum to discuss alliances, which never really took-off and which (since the game is ending soon) will not be too important now at Board:Alliances...
- Top Alliances
Why would you want to join an alliance?
By joining an alliance, you and your teammates are able to combine resources and reap the rewards of doing so. In particular, the alliance itself will be ranked for the time-limited Arenas. Once the Arena ends, all of the members of the alliance will receive a payout based on their overall-placement. The higher the members place in the Arena, the better the rewards.