If this is not what you are looking for, I must admit that I wouldn't know how to "group users" in an Oracle database. Though, a role might help. You'd create as many users as you want. You'd also create several roles (for example, those that can only select data, those that can update certain tables, etc.), grant required privileges to roles and - finally - grant roles to users.
I guess that it isn't exactly what you have on mind, but ... that's about all I can think of.