Sorry - I've not got any invitations available (had managed to get signed up early on, maybe a year ago, but because I was made bankrupt, don't currently have any credit cards and cashflow is still a bit unpredicatable this minute, so have not subscribed - not got a phone line so using broadband via Three mobile on my sister's account and usb dongle, but hopefully things will improve.
Just looked at 'how to get invites' and it says:
How do I get new invites?
Getting invites is one of the benefits of being a Premium subscriber in one of the countries where Spotify Free is available. You receive two new invitations each time your Premium account is renewed (yearly or monthly, depending on your payment method).
Looks like I'll have to go monthly, so there may be 2 new invites available each time
Maybe "bumping" this thread will shake some actual paying subscriber out of the woodwork who can send you invites...
Alternatively, Napster seems to be only a fiver a month... not sure how the two compare for 'completeness' though...