I have a client that needs a specific user management scheme. Users will self register and select the office they work from. Then a user manager from that office would receive an email that their has been a registration, and review, approve or decline. The usermanager would only be able to review users for their office. The user manager would not be an Admin.

The thought that comes to mind is to pull over all the User Management functions into a new module and set it up for role based access other than in the admin area. The one thing I have not figured out yet is how to determine who gets the email based on the office the user selected. Any ideas?

You're right on making the new module.  It's pretty easy to set up if you're familiar with .net, I did a similar thing for my company's site.  You can make your own login module (or modify the existing one which I would not recommend).  An if or case statement could be used to email the appropriate person.

If office1 then email manager1

else if office 2 then manager 2 etc.

On my setup I made a function to GetAdmin which will get the admin for the corresponding office.  Once getting the UserInfo object you can pull the email and send it that way.

I would use the security roles builtin to DNNand simply add a role for each office. Then add a simple page that accepts the user id, role id and a simple pass key as parameters.

http://www.company.com?userid=123;roleid=456;key=789

Then you can email the link to the manager, if they ignore the email nothing happens, if they click on the link, the user is added to the role.