How to add user to custom group upon registration

[Analogpoint]

It is an automated process after you have the promotions set up. Once an hour, vB checks the promotions and promotes the user automatically (it could take up to one hour after they register).

As an example, I have the Registered Users group set up so that all new threads/posts go to the moderation queue, so I can check them for spam (I don't do it manually, I use an akismet plugin ). Then I have a promotion set up for when a user has a minimum of 5 posts and 2 days of being registered, they are automatically sent to another group that doesn't have this restriction. I think you could do the same, just have the parameter be their email address instead of their post count.