@dansup
If you go by user count or even most recently updated, you'll fall into the same trap of most social media, which is promoting the groups that are already the most popular over the groups that are the most relevant. This leads to a handful of groups dominating, while new, interesting ideas wither.

I would suggest you promote groups whose tags (for the group itself and the groups posts) match the user's tags (for their profile, posts and boosted posts). Match by topic, not popularity.