The blog mentions that groups will have a presence in the game (probably some sort of meeting-place). At that point their purpose may become more apparent.
the glitch strategy group tends to have a large bump in activity around every test, and "new streets" gets used a ton .. but mostly for its in-game chat channel, not its forum.
one of the problems with the group forums right now is that you can't edit posts, so it is difficult to structure "informative resource" posts and update them over time. it is also difficult to click through every single group you're in to see if anyone has said anything new, and what.
groups seem to be a major work in progress. post editing and thread following tools being necessary for sustaining the existing functions and a number of more advanced in-game applications for groups being necessary to create new functions.