I'm afraid it’s not that easy to start a new group. Only the site administrators (mainly David Bruce and Anna Reynolds) can do that, and normally it depends on the level of demand, and whether or not an existing group could fulfil the role. Group members are expected to be active and to give feedback to the rest of the group. If they remain inactive for more than three months (usually – in Flash Fiction it’s a month because they’re very fast-moving groups) they’re removed from the group to allow someone else to take their place – so keep checking. Most groups are pretty fluid.
You have to apply to be a group host – although I've never heard of anyone being refused – the job is fairly relaxed, keep an eye on things, encourage other members – it depends on the group dynamics really.
Frequency of posting is strictly regulated across the whole site. You can only post one piece of work every two days at the most. This rule was introduced a while ago to keep posted work on the front page of the archive as long as possible. The length? There’s no official restriction but, in practice, the shorter the piece the more feedback you’ll get. It takes time to critique someone else’s work, and long pieces can be daunting.
Hope this helps. Like I said on your other thread – anything you want to know, just ask away.