If you join a Zombie Massacre match part-way through you’ll never be able to catch up with your teammates’ upgrade count, which, in my experience, means they usually kill everything before you can, making it even harder to get enough points for upgrades.
An easy fix for this would be to give late joiners some extra points, depending on when they joined, so that they can immediately buy a couple upgrades after the current day is over.
I think these values would be reasonable:
Joined on Day 2: 1500
Day 3: 3000
Day 4: 5000
Day 5: 8000