Allow the creation of customer groups, and allow invoices that can be
applied equally among all members of a customer group.

For example:

Say I am an accountant for a small private school.  Our billable customers
are the parents of our children.  We have parents who pay annually, by
school term, and by monthly (discounts for prepayment).  Parents can be
divided into the groups determined by how they pay.  As I create invoices,
they can be applied equally to the members of the customer group.  

I could create an invoice for the annual tuition and apply it to the annual
group.

I could create an invoice for the term tuition and apply it to the term
group as the new school term begins.

I could create an invoice for the montly tuition, and apply it to the
monthly group at the first of the month.

An invoice applied to a group would equate to an invoice to every member of
that group.  As tuition is the same for every student, the invoice would be
the same for every parent in the same group (which would make up the
logical reason for the groupings).


Thank you Warlord for helping me flesh this idea out on IRC.