Thanks for the reply, Marcos.
My experience with Roles comes from my (limited but growing) use of WordPress. They have pre-defined roles (Admin, Editor, Contribtor,etc.), and a plug-in that allows you to create new roles based on the the existing one. That facility doesn't have to deal with the idea of multiple workspaces, of course.
So, Groups and Roles are different, and I'm not sure what their relationship would be.
Maybe Roles are Workspace-based, while Groups are system-wide.
Perhaps there could be system-wide 'templates' for roles that could be customized for each workspace.
If Groups are to be system-wide, perhaps it is an expansion on the current "admin / not admin" that gets set per-user.
In this light, Roles would be linked to a workspace, while a Group would be a system-wide Role -- Admin, Editor, Reader, and so on.