We've been discussing the difference between roles vs groups. And I found the answer here
So let me quote from
"A frequently asked question is, What is the differerence between roles and groups?" Groups of users as the unit of access control are commonly provided in many access control systems. A major di erence between most implementations of groups and the concept of roles is that groups are typically treated as a collection of users and not as a collection of permissions. A role is both a collection of users on one side and a collection of permissions on the other. The role serves as an intermediary to bring these two collections together."