Thanks for the reply. I think I follow what you are suggesting and will give it a try. I will post again with news of my progress or if I run into additional questions.
The plugin is working great for us, but I think this would be an excellent feature to add in the future. When I first showed the messaging system to our client, their very first question was “how do we send a message to a specific user group with one click?” I also think it is better to select by a custom user field rather than a role – roles don’t always align with what business users define as working groups.