There are lots of groups and forums to explore here. I particularly recommend the relaxed, freewheeling Hang With Friends. In our current political climate, Writing to Congress (or calling!) can be very useful, since politicians ultimately care about reelection first.
Groups like California Atheists and Los Angeles are easy to bring back to life: just reply to a discussion (or start a new one) or just comment at the main page and say hi.
(Joining groups doesn't clutter up your "feed" or "timeline" on this site; some of us are in many dozens!)
And I keep discovering interesting things through the Latest Activity list in the lower right of every page. (In mobile view, it's the "Activity" menu item.)