Prom is, obviously, there to lend a hand and be his happy and bubbly self as he greets people. He's also more than happy to trust Shiro to keep watch while he bops around taking pictures.
Bill is on hand to help, too. I want to imagine he's mostly bussing tables and helping wash dishes. He can't really be trusted as a server, too much potential for him to trip over his own two feet and ruin someone's dinner.