Rename Additional User ID

Hi

I made mistake while adding user name/id under my accounts. Instead using our domain name for the email, I entered totally different domain name which is I don't owned. Why it could happen? Our company name contain two word and our domain name is the combination of those two words. But I accidentally use the first word as the user domain name which is I'm not the owner of the domain.
Could you help me with this situation?
My suggestion for the future is to have the additional user to activated his/her own accounts from their registered emails.

I know this my own fault but I look forward some solution to figure this out. At least if I can changes his/her user name/id.

You have a great program out there and I really satisfy with the feature that you guys provided.

Please help...