@Kyrne, I know you have done a fork on the Davis Invite Only extension. I think this is a potentially powerful extension for forums with many user classes, and I would love to see the ability to send codes that automatically add the new user to a certain user group.
For instance: You send a code to an email list that contains all of your club members and they are enrolled upon signup in the "member" user group, with access to members only tags.
You send another code to a group of past members, and they are enrolled into a "past member" user group that does not allow them access to member only tags, but other tags are available only to them.
This could save a great deal of moderation if user groups could be set upon signup. These codes would also have to be capable of multiple uses, I reckon.