Friend email invites are coming in a couple of weeks, nice to hear that it is a feature people want.
From a security standpoint it is a bit tricky since preferably the invite would allow the invited friend to connect to you. And since email is clear text some type of additional security would be required.
We are currently looking at "shared knowledge questions", basically it means: first time you connect to the friend that invited you, you are required to respond to a question, like, "what color is my car?" or "how many beers did I have last night?"
For the extra paranoid we will support 2 factor auth, meaning that in addition to the email we also send a code over a separate channel (like sms).