Drop down has users of a certain role

I assume you mean the actual users assigned under the 'Users' and not a table of roles and users that you setup yourself? For Application roles and the associated user in that role you'll have to sync the Application User data into a new table using Pipelines and the API. There isn't a native functionality that grabs users and what role they're in within an application. I'd suggest taking a look at API_UserRoles and API_GetRoleInfo as the combination of those (2) calls will get you the roles in the app and users in each role.