There was a redesign of the site that took away a sidebar on the Forums page showing new groups which I think used to be how groups got more members and interest. I miss that. There is a Forum Jump - if you scroll way down you can find the Groups main page that way. Or just click on the sidebar on the right to go to any group there, and then at the top you can find the Social Groups link to see the whole list of them.
I’ve always enjoyed the groups but they’ve slowed down a lot in the past 2-3 years. It could be they’re harder to know about and get involved with due to the site redesign. It could be the influence of Facebook or who knows what.
When the groups were in their heyday, as I think of it, there were lots of really specific ones being started and they would gain members fairly quickly and have good participation. But the general talk ones and a few of the prayer ones have usually been the largest and most active. They probably have more of a mixture of religions in their memberships since they’re more generalized.
I’m in some of those but the topical interest ones are my favorite, usually. And some of them are more Catholic in tone (“Catholics Who _____”, etc.), though no one will mind a non-Catholic joining in as long as they are okay with the group talking about Catholicism and don’t try to attack the Catholic Faith, which rarely happens in my experience.