As a mastermind, can I dominate multiple enemies at once?

Solution 1:

The rules for this were changed 5th of March 2014 (emphasis mine):

Every player with the Dominator skill can now dominate one guard each at the same time. But apparently the old pager operators got fired, and more experienced operators are taking over. So clear your throats, and get ready to answer the pager as soon as you dominate a guard.

So one dominated guard or police officer per player with dominator. That means no, you cannot dominate multiple guards at once. You will need a friend to help.

Before this change the rule (which console and unpatched versions might still follow) was one dominated guard or police at a time. Want another one? Kill the existing one.

Solution 2:

The video you were watching was in the beta of payday2 there you could dominate multiple guards without a problem. But they nerfed the perk, which now only allows 1 guard to be dominated, after 1 guard you will not be able to dominate another unless you kill the dominated guard.