They require your first born child, but hey, if you've met my first born, you'd know that isn't always a "downside". 
Are there any downsides to joining groups on chess.com?
From the perspective of the community as a whole, they fracture the discussion, but then on the other hand there are a lot of groups who's discussions I'm grateful not to have to sift through in here.
Aside from conversing with like-minded individuals who have common interests (depending on the nature of the group) the biggest benefits are probably vote chess and team matches which can only be played by members of groups.
From the perspective of the community as a whole, they fracture the discussion, but then on the other hand there are a lot of groups who's discussions I'm grateful not to have to sift through in here.
Aside from conversing with like-minded individuals who have common interests (depending on the nature of the group) the biggest benefits are probably vote chess and team matches which can only be played by members of groups.
I'd describe it as a self-sorting mechanism.
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Every now and then I get invited to join a group at chess.com. What is a group exactly? Any risks/downsides that I should be aware of, or is it save/reasonable/OK to join one, a few or many?