You can add a user (as noted) in the Configure >> Users section.
You can select what level of access they get (from Admin through Read only) as part of that process.
To remove a user, click the box next to their data and click the Delete button above.
Given Admin level changes inside WMT accounts are infrequent, this is how the system is set up. To edit a user's info, simply add them again with their fresh info (in this case, sounds like a new email addy is needed)
Now, if you're referring to the login information used to login to WMT in the first step - that's a Live ID and controlled by Live ID. Changing that information is agnostic of the data inside the WMTs themselves.
So, if it's the LiveID email at issue here, you'll need to reverify the website as starting a fresh WMT account will trigger a new verification code be generated. While this might seem drastic, in reality, once you replace the verification code in the <head>, the data associated with the domain remains intact, so all historical data for the domain will appear in the "new" account opened under the new/different LiveID.
That should allow you the adjusted login I think you're looking for, and keep the data intact for the domain. ;)
Hope this helps Ken, though we do realize it may be more work than you originally hoped for.