Add to usergroup by email domain

[garthcoleman]

I need to setup several private forums.

I could password protect each one, but I would like something more intricate.

Based on a user's email domain (or certain email strings), I would like to automatically add them into a specific secondary usergroup. Membership in that usergroup would allow access to the particular forum(s) based on admin settings. I also need this to happen on existing users, not just upon registration.

Does something like this exist?