I have turned it off a few times. When you recieve an invite the box pops up and there is a small box saying "never recieve this message again", or something like that. It is easy to click that on accident. I have done it a few times, once I receiving an invite while in the middle of clicking something and I clicked the box on accident.
Easy mistake. I didn't even know I did it until the next time I recieved a group invite and the box did not who up.