User blog comment:Royal00000003/Update 6.0.5!/@comment-24953058-20140519051553/@comment-10824019-20140519143928

You can, but GameCenter doesn't let you invite friends into existing matches that your friend may be playing. Plus, they'll know you're playing a game on GameCenter, they just won't know what server or mode you're playing on, or even if you're playing online at all!