Everyone close your eyes...seriously!
Ever wondered how to take a group picture where one or more people aren't blinking, or with partially shut (droopy dog) eyes?
Or one where people don't have different shapes of mouth movement.
For the first problem, I simply ask them to close their eyes. After I've convinced them that I'm serious, i simply tell them to open it and then take the picture immediately. This is because no one blinks immediately after opening their eyes.
While I'm at it i also take a picture of everyone with their eyes closed, cos frankly it makes for an intriguing shot.
For the second problem (the mouths) I ask them to say the cliched, overused, cheesy word...you guessed it...'cheese'. 'Cheese' was chosen by the sages of old because it contorts the mouth into a smile-like shape. 'Cheese' was also useful in the olden days because it's irrelevance to the event made some laugh.
In the end it doesn't matter what word is used as long as it's monosyllabic and does not require more than one mouth shape to produce.
Cause if you choose to say 'Jesus' for example, you'll need to be more careful not to have a shot with some people mouthing 'Jee' while others are mouthing 'Zis'. That in itself is not necessarily a bad thing if you wanted a more photojournalistic approach of the group shot. In that case you shouldn't be bothering with uniformity in the first place...just click as the shutter-finger itches.
Ps. Try this for size...get everyone to say 'Moo'...while closing their eyes!