I had invititation(s) in the begining when beta-accounts could invite, but now only the premium can so I can't help you. I guess you must find someone with an premium account who can invite or buy the game.
PS: I got a second account if you want it, let me know. It has a little bit big leaver percentage, something arround 10% (which is not very bad), but you'll still be able to play.