Self-mantained User groups?

First – wordpress miss user groups at all, but it can be somehow “fixed” with role manager plugin and post levels plugin, where you can make special level and then asign it to one or more users. But its bit stupid, too much manual work, not much functional. And i must say, missing of groups it is the biggest disadventage of wordpress comparing to other systems.

I would prefer real groups system, where you can just simply asign each user to group(s), allowing group posts – so posts, which can be edited by anyone from selected grou(s) etc. And it should be self-maintainded – i mean – each user should be (optionaly) allowed to invite anyone other to register (if not yet registered) and automaticaly join group, he is invited to. It should be also possible to name such groups, give it descriptions and other notes, show members from each group, show groups of the user in his template, show articles made from that particular group, special group roles (group administrator) etc.

I have read many forum topics quoting user groups, but do not found any such plugin. Does anyone developing this, or is something like this in plans for new version?