RancidLunchmeat
Veteran
That's what I thought too, a few months ago, but the feature has been removed ever since, I think. Not 100% sure. But I got the invitation directly from Xbox Live, never from a friend.
My understanding is that friends can essentially "promote" people for invites, and then the invite comes from Live itself. I never got an invite from a friend, it came as an Xbox Live notification.
When I became a preview member last week or two? (THANKS AGAIN!) I sent invites out to everyone on my Friends list that I was pretty sure wasn't already a member.