- What is the nature and description of the request? 
As an user, if I know that a group (or user) exists, and have permissions to see the groups/users, but make a typo when adding a role to the group/user I should receive a warning that the group/user does not exist, potentially receive the option to create it anyway, but I should at least receive the warning.

- Why does the customer need this? (List the business requirements here) 
End users will be able to add users/groups to their project in any number of roles, however typos do occur and this warning will save the system admin's a lot of trouble caused by mistakes

- How would the customer like to achieve this? (List the functional requirements here) 
Something along the lines of checking if a user has permission to do something, just create a warning if they have permission to but are giving a role to a nonexistent group/user

- For each functional requirement listed, specify how Red Hat and the customer can test to confirm the requirement is successfully implemented.
Attempt to add role to group that doesn't exist

- Is there already an existing RFE upstream or in Red Hat Bugzilla?
Not that I could find.