I just created a project so my Solution Explorer looks like this: 
(source: kalleload.net)
But I want to add another project which is related to this project but it’s going to be a class library and it’s going to output a DLL that I could use in any other project.
So how can I create a ‘solution’ with multiple projects?
Just a related information:
There is also an option called ‘Always show solution’ (in Tools/Options/Projects and Solutions). When enabled, the solution is always shown, even if there is only 1 project.