I am able to assign a user to a user group using the User Admin in Apex.
I don't know how I would be able to assign a role (that I know how to define that for an individual user).
The only thing I can see is a name for User Group and a Description!
My requirement is to define a group of people to be assigned to one group/role, so that every change to that role can be automatically be applied to each user in that group.
You create groups and assign users to those.
Then you need develop authorization schema that utilize user group information.
I'm not sure do I understand question/problem correctly.
Also, it is better post APEX related question in Application Express forum