Regeristing into a certian group

[KWH]

I am looking to be able to require people to register on the site into a group based on an access code they are given at an event tobe able to view all albums from a particualr event.  I can set each album to private and give them all the same access code by event.  This would require the guest to login to each album to view the images.  There can be as many as 100 albums per event that everyone that attended that event would have access to. I would like to ba able to issue an "event code" that would be require when registering.  The "event code" would correspond to a group already created on the site and the guest would then be able to see all albums for the event.

In my head it looks like this:

Registration Page: All Fields Required
User Name: bob
Password: *****
Re-Enter Password: *****
EMail: Bob@bob.com
Event ID: 12345

Upon submision user Bob would become a member group "12345"

I have checked the forums and have found a few items that are close, but would still require the Admin to move the user into the correct group.

Thanks in advance for your help.

Keith