I wanted to add a Blackberry admin user in the version 188.8.131.52 web gui but it didn't go as well as planned. The fields to fill in aren't as intuitive as I thought. It is confusing to know which field is for your new admin user you are adding and which ones are for your actual BES admin user
The documentation isn't too helpful either with meek suggestions as "...In the appropriate fields, type the name and authentication information for the administrator..."
After trying too many combinations than should be necessary and, getting the following error - "The specified account is already assigned." way too many times. I eventually figured it out.
The two User Name fields are not described very well and their location leads one to put the wrong value than one would think.
Display Name: is the name of the new admin user you are creating - use anything you want here
Authentication Type: I used active directory
User name: The BES admin username - NOT the user you are creating
Password: The password for your new admin user
Confirm Password: again, the password for your new admin user
User Name: The username for your new admin user NOT the BES admin user
Administrator Password: Your BES Admin password
Here's the form filled in with descriptive values.