Well, if you're not going to divide XP, yet there's a sharp drop when you hit seven party members, clearly you either want to bring exactly five friends or a full-blown horde.
Here's how this system would play out at the table:
Scene: Solace. The Inn of the Last Home.
Goldmoon: "I bear the Blue Crystal Staff that everyone is seeking! Please, brave heroes, help me escape from these evil nogoodniks!"
Tanis, Sturm, Caramon, Raistlin, Flint, and Tasslehoff perform a quick headcount, look at each other, and order another round of beers.
End of campaign.