[To quickly switch between document groups try Task Canvas]
The free Favorite Documents extension lets you create links to frequently used code files and then quickly open them as a group or individually from the Favorites menu in Visual Studio 2015, Visual Studio 2017 and Visual Studio 2019:
In addition to the Favorites menu, you can access favorites from the Favorite Documents tool window. Solution-specific favorites are separate for each of your solutions and stored next to the solution file, global favorites works for all solutions.
You can create links to folders, projects and other items in Solution Explorer, create links to folders in Source Control Explorer and you can create links to solutions.