The sentence is said to appear in a textbook.
I feel this is a cleft sentence to emphasize 'them,' but then it's omitting 'who are' before the 'waiting,' but is this way of expression common?
Yes. While inserting a "who are" would still be proper, the form as it is is more common. The emphasis can be a slightly split if the sentence is referring to the opposite of the norm, which I can't tell from the current context.
An opposite situation: "Usually it's me flirting with the ladies, but since I got this sports car it's backwards."
A common situation: "As a priest, when people come to confession usually it's them doing all the talking to me."