Adding Admin Users To Sub MultiSites

I have a main site, and then child sites on an WPMU setup. I want the admin's of the child sites to be able to add more admin users (I'm using the pro sites plugin to sell value added services). Currently you can search for and add a user with a current account, but that's very clunky. Ideally I would like the admin to be able to add a user, and specify their email. The email would then give the user the login and password to log in with.

Anyone know a plug in or other way to accomplish this? Thanks in advance!!!