custom permission structure

 I've been trying to get sorted for weeks, But it might be one of those things where there is no correct answer, and a few ways to go about it.
I have a complex application that allows users to set up and manage Projects amongst other stuff, and soon I hope to add a custom forum to the application.

I need to allow users (project admin) to assign permissions for users on the project.
 what I thinking was for a table structure:
tbl_users_in_groups: GroupID 1 to Many UserID
tbl_roles_in_groups: GroupID 1 to Many RoleID
tbl_permissions_against_role: RoleID 1 to Many Permission_type
  

does that look right, i havent managed to find any good documentation on security structures like this
 any help/ comments /links/ articles appreciated .
 Lex
 
 
 
 
...