Mat does use Gmail, just not very much on a day-to-day basis. Same goes for me (and I've got 1078 people in my contact list on Gmail) and I've not got an invitation either. I think it's partly to do with whether you've linked your Google account to your Twitter or Facebook accounts. In my case Google keeps saying "Are you @stealthmunchkin on Twitter? Go on, please tell us you are. We know you are. Please tell us." And I keep saying "None of your business". I suspect if I just clicked 'Yes' on that, the invitation would come pretty quickly.