Hi, Tamara-
You can sort of do what you're describing as long as you never have more than two levels of roles. It's not as nice as being able to assign Role Groups to other Role Groups, but you may find the approach helpful. If you need to set up more than just a few roles, the following info should be helpful...
Managing lots of user roles in DNN
The trick is to think of individual "Security Roles" as "child groups" (groups of users in those roles). Your child groups (security roles) must add a prefix to the role name to reflect the child's parent role group. Really this is no different than how anyone would normally use role groups except that you're adding a prefix to each security role to reflect it's parent role group instead of just relying on the DNN "role groups" feature to define which roles belong to which groups. Here's an example to illustrate what I'm talking about and why the prefix is important:
Using your Communications Group example, it would work like this:
Parent group -> "Communications Group" = a role group named "Communications"
Child group -> "Communications Leadership" = a security role named "COMM-Leadership"
Now, to make it work, you assign all security roles beginning with "COMM-" to the "Communications" role group. With that set up, when someone gets the "COMM-Leadership" secuirty role added to their user account, that user is automatically added to the "Communications" role group. Now, as John said, the "Communications" role group has no security permissions attached to it -- role groups are simply a way to help organize security roles. You'll still have to configure all appropriate permissions for each security role (i.e., there's no role inheritance of permissions from other groups/roles). See the link at the bottom of this response for a few other reasons why the current "Role Groups" implementation is not very useful.
Why use a prefix in the Secuirty Role name?
The need for the "COMM-" prefix becomes apparent when you're ready to set up roles for another role group. For instance, if you created a similar Leadership group for the "Sales" organization, you'd set things up the same way:
Parent group -> "Sales Group" = a role group named "Sales"
Child group -> "Sales Leadership" = a security role named "SALES-Leadership"
Using a prefix on the role name ("SALES-" and "COMM-") insures sure you end up with separate "Leadership" roles for the separate groups but are able to use the same name ("Leadership") for the same type of role.
A Better Example
Here's a recent thread about roles and groups in general terms and with a better, more generic example to the one I gave here. It also cites some additional limitations of role groups:
DNN Role Groups and Roles (which one should I use)?
http://www.dotnetnuke.com/Community/Forums/tabid/795/forumid/108/threadid/270383/scope/posts/Default.aspx
-mamlin