best I can tell:
they are a mini tournament between 2 groups. You only "hurt" your group by losing if they are extremely serious about it, most are NOT (I do not know of any who care but someone out there probably does).
The top rated people who join are paired. If there are extra on one side, the lowest sit out, best I can tell. Some tournaments are rating limited, like under 1200 tournament I could not join, or over 2000, I could not join that either. Open is for anyone.
Join them, play, have fun. That is why the groups exist!
I joined one of the groups. Now I'm getting "team match registration" notices in my inbox.
Do these matches work just like any other game or tournament? Do I hurt my group / team if I sign up and lose? If it's just my rating, I don't mind taking on some higher-ranked players and losing more than I win. But I wouldn't want to take a spot on the court when I should be on the bench, so to speak.