TinySpark has turned off invitations for a while now as the redid quite a bit of the "leveling system". They have commented that the invitations should be opened again in the near future but no one knows exactly when.
I've been waiting for invite from the official site for couple of months, starting to think that I'll never get it. If anyone has some extra invites laying around that needs to be spring-cleaned, I'd love an invite. My G-mail address is rolle83.