"Role Groups" simply act as containers (and sometimes as list filters) for groups of individual roles.  Security settings are still administered at the role level, not the role-group level.

...So Why Have Role Groups?
An old "ROLES roadmap" included the following:
Role Groups - In large websites the number of Roles to manage can become unwieldy.  This enhancment would allow roles to be grouped for convenience in roles and permissions management.

Without role groups you'd have to adopt naming conventions to keep track of related roles.  In your example of "talent group", you might adopt "TG" as a prefix for your various talent roles:  TG_ACTORS, TG_DANCERS, etc.  By having the role group "TALENT" you don't have to worry about sticking to a naming convention for the individual roles within the "TALENT" group.......or do you?  Unfortunately (in DNN 4.x, at least), when you use the USERS->MANAGE ROLES interface to add roles to a user account, you don't see ROLE GROUP info (nor do you get a chance to filter available roles by selecting a role group).  Rather, you see all of the individual role names in one big list.  Ugh.

All Role Names Must Be Unique
Role groups don't provide unique namespaces for the roles they encompass.  Because of this, you cannot use the same role name twice (even under different role groups).  This greatly diminishes the usefulness of role groups.  Example:  You can't create role group "SALES" and role group "IT" and then create "MANAGER" roles under both "SALES" and "IT".  To do so you'll have to resort to naming conventions for your roles such as "SALES_MANAGER" and "IT_MANAGER".


Benefit:  Filter Permissions
Fortunately there are a couple of real benefits with the current featureset:  you can assign user permissions to pages and to modules using a "filter by role group" option to filter the list of roles to a particular role group (though you'll still have to click each role you wish to assign VIEW and/or EDIT permissions to).  By default the displayed role group is the "Global Group" -- you can also display "All Roles".  This filter can be a valuable feature if you have a lot of groups and have not employed a naming convention for roles.

Benefit:  Custom Uses
You can leverage the role groups feature in the logic of your own custom modules. 

Examples:

• Email all users of a selected role group.
• On 1/1/2009, disable accounts of users under the "2008" role group.
• Administer online polls/voting/contests based on role group.

Perhaps we'll start seeing more and more modules that make good use of the "role group" feature.  So far I've not knowingly used a module that was "role group"-aware.

There may be additonal uses of the DNN 4.x Role Groups feature that I've overlooked.  If so, I'd love to hear about them!

It's ok -- I still have 2849 posts to go to match Chris' contributions to the DNN forums...