So I'm viewing this as the therapy sessions. Characters might recur but not outside of group. I can see the appeal of EVEN MORE CHARACTERS being introduced but honestly, sometimes you wanna have a group outside your group to talk to about things. Having your support group bleed into your friend group kinda defeats the purpose(for someone with a well established and pretty supportive friend group).
On a meta level, I think this is the perfect way to handle the situation. As it is, certain characters fall out focus for months (or even years) at a time, so I don't think the principal cast needs to get any bigger. I see the therapy group, with the possible exception of Arthur, being occasional "guest stars" with minor roles.
In-universe, I think it works out for Faye, too. She doesn't have a lot of people she can turn to at the moment; she's a long way from her family, she obviously isn't close with Dora right now, she never formed close friendships with the other CoD employees, Angus is now out of the picture, and while Marten is supportive, he's just started a new relationship. On top of that, Marten's not really cut out for the tough love or assertiveness that Faye needs right now. Hannelore definitely is, for all her own issues, and she's clearly willing to help, but she's just one person. Perhaps meeting other people who have had similar struggles, in an environment where she can't openly snark at or bully them, is exactly what Faye needs to at least take the first step in healing and dealing with some of her issues.
I'm still guessing we don't see any familiar faces at the meetings, but I could see Faye and Hanners running into Sven at random on the street after
leaving a meeting...probably right after Faye says something about how she felt the meeting helped a bit and she feels like she's made progress.