What the group allows you to do is interactively debug a system that inludes vb DLLs and such. If projectB is a dll, the debugger will drop into it when a call to it is made.
It doesn't allow you to share forms between one app and another, however. To do something like that, forget about the group and add formX to project A instead. Project A then stands on its own. If it's a common re-use scenario, then consider making an ActiveX component (dll or control) that you can reference from any project.
Hope this helps.
--Bob