Dorothea, All of my layers are in one service. I have tables in the database which dictate what users have permissions to view. When the user logs in, I query the database and get a list of what we called "Result Groups" that are the logical groupings of layers. These values are what you see in the dropdown. On the client, there are objects called ResultGroups that I use to interact with the other elements such as layers and their related business data tables, along with the reports, images, scanned documents, etc. You could probably populate the dropdown with the layers themselves, or use group layers in your service. If you open the application and look at it through Firebug, you can add breakpoints and follow everything that is going on behind the scenes.