This is usually caused by corrupt outfits.
If you're not opposed to mods, I would suggest to use either Overwatch or ErrorTrap for help with that as you seem to have quite a few.
Another option is to take those Sims to CAS and change their outfit but this can be a bit difficult if you don't know which ones they are.
Hmmm. odd. Since it is happening to some resort employees as well, it would mean that maybe a hairstyle or hat style is corrupt?
Can the game pick styles from EPs that are disabled in the launcher, even though those EP were never in play for this particular game? it was a new game and I disabled a lot of EPs before starting.
If it was a new game, it shouldn't but with Sims 3, you never know.
Here's a few tips: http://sims3.crinrict.com/en/2011/04/bug-invisible-sims.html
