I make group customs for different friend groups -- a double date group, a workout group, a work group, etc. and that's been working really well. It also helps if the game hosts community events where people can interact -- whether it's an actual event for people to meet up at (we have a musician who played a show recently and made an in-game event post), or a free-for-all type post where people can go nuts and talk to each other.
I also shamelessly comment-hop, but if it's going to last more than a few comments, like someone else mentioned, I will move to a custom or the person's friends-only post or something -- and it is courteous to leave a comment on the original post as well, and not just respond to person B in person A's post exclusively (ouch!)
What I appreciate in fellow players is a willingness to interact and engage with people outside of just their storyline partner, an openness to connecting. Personally, I try to find something to talk about with everyone, rather than just sticking to the characters that I already know... sure, you might not click with every character, but you can strike up surprising friendships that way.
If you're looking for a small but active group to test the waters, beaconrock is a great choice. I stopped writing for a few years and just came back recently, and it's been the perfect place to ease back into it because it's so friendly and fun. Let me know if I can answer any more questions! <3