After a few months of Groups we have seen that making everybody go through the Wikimedia User Group process (or any process) won't work, no matter how light that process is. Still, everybody agrees that grouping the people interested around a topic is useful...
The new formulation of Groups tries to address this. Wherever there is a tag there will be a Group listing the people related to that tag. How active and structured a Group is will depend on the activity of the people around it: it can go from zero (with zero cost) to an official WUG organizing local events, with all the shades in between.
The basic functionality might be provided by the Category features, or at least they have many common similarities.