How do I build a beta invite system?

Hi! I'm getting ready to release a beta for my rails app and I was wondering if there is an easy way(perhaps a gem?) to build an invite system, so that only users that receive an invite through email can sign up on the site? Existing users should have the ability to invite friends.

I haven't made an episode on this yet, but I need to. I use https://github.com/scambra/devise_invitable for sending invitations. Basically it lets you create user records without a password, it invites them with an email, they can click the link to set their password and get setup. It's really nice.

I use this for the team accounts on GoRails which work similar to beta invites, where a user can invite other people.

Thanks Chris! That's just what I was looking for!