Oooh, I like the ponytail holder idea!
Every year, we are either a party of 7 or 9, depending on which grandparent/aunt/uncle combination tags along, so the resort refillable mugs become quite the scene.
First, we usually stay in a villa. I do the dishes every night. (This was nothing short of comical our first year in regular hotel rooms at WL. Yep. I filled that bathroom sink with dish soap and washed dishes.) This makes things a *bit* easier, as each person's cooties is washed away by my incessant scrubbing (usually after midnight, while waiting for my clothes to dry and trying in vain to hide the plastic Cinderella sword from my son while he sleeps...but that's neither here nor there).
Second, we assign lid colors to groups. Last year grandparents were orange, girls (including mom) were purple and boys (including dad) were black. This cuts down further on the cootie factor. The odds that you're getting your lid (or sharing from a smaller pool) are better, especially during the day when they're clunking around on the stroller.
Lastly, I am that annoying Sharpie user. I do write our names on the bottom of the cup, typically in hilarious (to me) fashion. This helps keep cups organized and ensures that my husband isn't embarrassed in front of 30,000 park visitors while drinking from his STOLEN FROM DADDY cup unless he tips it high into the air.
Usually it's just my family of 3, so we've never had a problem getting different coloured mugs. 
