Yep, Premium members do not need an invite.
It's brilliant (esp when combined with Airfoil - millions of tracks in every room that has either a PC/Mac/LinuxBox(TM!) or iphone... genius), and I wouldn't be without it. Premium comes into it's own if you have an iPhone/iTouch where you can use the Offline Playlist function. Perfect for in the car.
Though there is one downside with using the Spotify/Airfoil combo. My daughter has worked out how to get Xmas songs playing in 5 rooms now (Mac in main system, iphone/dock in Kitchen, AE in lounge, PC in study and iTouch/dock in her bedroom)...there is only so much Slade one guy can deal with!