In Heart Mercedes says Quinn is just rejoining the god squad and Joe just joined, does that mean Sam and Mercedes have been spending time together in god squad?

It could've been a possibility. It's uncertain, though. The God Squad may have been active since Season 2, but we just couldn't see that. Maybe somewhere around "New York" or something, 'cause that's when we started seeing Sam and Mercedes together. Quinn could've been a part of that a little bit before that, and then quit prior to "The Purple Piano Project" or so.