I was able to invite people about two weeks after getting my account. I started out with three invites. Handed those three out, got three more. Handed those three out, got five more. Handed those five out, got five more. Handed those five out, got five more.
My friends who I gave accounts to about two weeks ago, recieved invites about 3-4 days ago and they each got 6 invites to give out.
pushlatency, they don't show up in your inbox. its a link that shows up under the "search mail" button. I missed it for a good few days, it doesnt pop out at all.