UserName is often used by clients who have the Single Sign-On option.
Instead of logging in with MSmith@school.edu, I can then log in with MSmith. UserName is not required for Single Sign-On; some clients have a standard of using the email address for their login process. This is preferred even for single sign-on as it allows users to log in from http://www.regroup.com/signup instead of requiring the subdomain such as in the embed or at http://schoolname.regroup.com/signup .
For clients who do not use the Single Sign-On option, you can also upload a user name for members to use when loggin into Regroup.