As it is stated in the Rules on main page of the group: you have to be a member of Main Giveaway group to stay in the GAntagonist group. Let me explain this: GAntagonist group is a general group where you can create as many giveaways as you want with region and level restrictions, lasting for one hour or a week and you don't have to create your GA when you will win something. Generally you can do what you want.

Main Giveaway (MG) group is kinda sub group where we are doing our main thing: if you win sth there you have to give back. This is the group where all the rules really applies. I had to create this sub group because people wanted to give away more games than just one every 2 days :) So GAntagonist group is for any additional gibs while MG is for our main giveaway.

Maybe it is statted more clearly in FAQ discussion in the group. You can find how to apply for MG group in the Rules on the main page :)

Oh, ok you've already found it ^^ Invited :)