Microsoft has been very Scrooge-like with their beta invitations. To start, on their
official blog they began deleting requests for invites to beta test their latest IM client, Windows Live Messenger. Those who were accepted to the beta program didn't get any invitations to share initially, and when they did, there were very few (I've gotten two so far).
There have also been problems reported with people receiving invitations in Gmail. The download "button" that appears in the invitation is non-functional if you receive the invite in Gmail. There is a workaround (view source, find the URL, cut-and-paste), but it's a long way from polished.
I'm hoping the team starts opening the door to the beta a wee bit more. 10 invites would be nice; 50 sounds even better.