You don't join FB groups to market to them! You join to do customer development. There's a huge difference between going in and going "try my app" and "hey I'm

You don't join FB groups to market to them! You join to do customer development. There's a huge difference between going in and going "try my app" and "hey I'm an app developer, building this thing, and I'm wondering if people here would find X or Y more useful". Do it the first way and you'll get banned. Do it the second way and you'll get people asking to try it.