I have been using Google Voice for probably about four months now. I received three invites to give out from Google about two months after I started using it. Sorry I gave them away already...
Look at the menu on the left. Below your calling credit, I had a box that says "Invite a friend (3 left)" or "Send an invitation" or somewhere along that line.
I've got GV but haven't messed with it much, sorry no invites though or I'd send. Worst case scenario there's people actually selling invites on ebay lol
I just got 3 invites today. I'm asking if any of my friends want one but if they decline they're available. PM me and I'll let you know what's going on.