I'm going to sidestep the question of a group charter, and ask for something more concrete and shorter term from this group.

How should people that aren't defining the standard library use modules?

I believe that wg21 should provide some recommendations, though those recommendations shouldn't be in the IS. Note that these aren't requirements... just guidance for some definition of a "typical" project.

If we don't provide a recommendation, I fear we will get a huge set of different styles, making build systems and dependency management even harder than it is today. Here are some more specific questions:
* What granularity should I provide for my modules?
** Do I translate headers to modules one-to-one?
** Do I just have one big module for my entire library?
** Something in between?

* What file extension(s) should I use for module files?

* How should we map module names to file names? Just by replacing dots with slashes?

* How should tools find modules?

I know just enough about modules to ask subtly wrong questions, so feel free to refine these questions. Regardless, I think some wg21 associated group should provide guidance, and I think this is the group to do so.