You can clone with
HTTPS or Subversion.
There should be a more flexible way to contact other translators / teams. Maybe this request should be linked to the more general request for an associated forum board
From the addon author point of view, the forum board is a nightmare: I have to check two different places (the translation page and the forum topic), I always miss new replies, and I'm not even sure everyone reads it. What about a system of announcements that can be sent from the addon author to his team, and an a way for translators to ask questions to the addon author and/or see replies?
First one is already there, which looks much like old notification system. You enter subject, message, hit send and all your translators get the message on email.
Second would require more work. And it is worth checking out existing Django solutions here.