A traditional forum for communicating. This is the forums operated by the MegaMek team. There is a MegaMek subforum on the Catalyst forums as well: http://bg.battletech.com/forums/index.php/
A team communication tool enabling real-time chat. The public slack allows member of the community to communicate in real-time. For more information see http://www.slack.com
A twitter account for the MegaMek team.
A facebook group for MegaMek.