The Sirloin Staff are waiters who work at The Singing Sirloin. They sing to their customers songs depending on the occasion. One of them is called Mary Kay and she is at the front door of the restaurant aware for customers who want to escape, like when the Simpsons were getting away to not pay and the waiters sang "They're getting away, stop them Mary Kay!".
A different group of waiters are seen singing different songs for different customers in Life on the Fast Lane; One group had sung an anniversary song to Mr. and Mrs. Winfield, another group sung a pregnancy announcement song to a couple, another group sung a funeral song to some people, and another group sung "Happy Birthday" to Marge. The waiters were seen again in "Homer vs. Dignity", where another different group of waiters sang a "Happy First A" song to Bart.