Put the group in pairs.
Each pair has to count up to 3 alternatively.
Phil: 1 Mandeep: 2 Phil: 3 Mandeep: 1 Phil: 2 Mandeep: 3 Phil: 1 ... etc
Once they've done this for a while, introduce the following:
Each of these ups the difficulty.
Even at its base level this exercise is harder than it looks.
Each new rule introduces a moment of decision-making in your mind - and each decision has more options than the last
Once you've named a person or made a sound, you can't use them again.
It disrupts the normal rhythm, you have to be in the moment.