How to group users per role?

I don't succeed at grouping users by roles, one role at a time.

Take 2 fields:
- user
- roles

Take this set of user roles:
- user1 : roleA, roleB
- User2 : roleA
- User3 : roleB

I wish to get the following output:

RoleA
- User1
- User2

RoleB:
- User1
- User3

If I group by role, I get:

RoleA, RoleB
- User1
RoleA
- User2
RoleB
- User3

Thanks for help