I was one of them till recently when I found out that even other users of the group buy can find your niches.
I knew that the admins of the group buy were able to do that, but I had no idea that even other users can do it.
Probably you think that you are saving some bucks, but actually, you don't realize that you're losing.
-You make your niche more competitive, which means you have to spend more on backlinks.
-You get much less income (even if you rank on the first page of Google) because you have to share with your new competitors.
I just figured out that when I saw new competitors popping out to a golden niche that I have found.
TL;DR: Don't fucking dare to join group buys as in the long run you will spend more rather than saving.