The next time one arrives, you can block that group from sending you any more.
Also, you could change your privacy settings ... (My HOME -> Account. Privacy settings).
INVITES
Control which kinds of invites you want to allow/receive.
FRIENDS: allow others to send me a friend request
GAME CHALLENGES: allow others to challenge me to a game
JOIN GROUP: allow others to invite me to join a group
TOURNAMENTS: allow others to invite me to a tournament
Why do I keep getting invited to groups? In the last two weeks I have been invited to "Child's play" at least 3 times. I know active members get a lot of invites but what I don't know why it just started a few weeks ago.