I started one (with BN) support. After a year I had about 50-75 members. I underestimated how much effort it would take to recruit members and post stuff to seed the giving, so the group never was very active.
They have a lot of strict rules (membership boundaries, can't post info about free community events, etc.) I'm sure they have good reasons for all the rules, but it felt like I spent most of my time telling people "no." I stepped away after a year as an admin, and it doesn't look like my group is active anymore.
I'd join an existing group, but I would not be an admin again. YMMV, though, because I may just be too much of an introvert to enjoy the interactions needed.
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