I was taught "point at someone and tell them 'you, specifically, call 911'" but I like touch a lot better. Makes more sense for getting someone to actually do it.
Touching someone when asking them to do something is a well known psychological trick. It immediately increases intimacy, which makes the person more likely to do what you ask. Though it can also backfire if the person is particularly sensitive and finds the intimacy uncomfortable.
I had this directive as well.
I was told to point at someone and say, "YOU. Call 911."
I've found "You, what's your name? Steven? Steven, can you call 911 right now?" to be very effective. It breaks the ice with a very easy task. It can also help later when you need to need to find, describe, follow up with, or thank the person.