I am pretty sure that CMs are required to take a Guest's pic any time they are asked. Maybe some CMs can confirm or deny that.
Whenever I ask a CM to take a pic for me, or more frequently, when they volunteer, I simply ask them, "Do you know how to use one of these monsters?" as I hand them my camera. If they are familiar with SLR cameras, they say yes, and if they're not, they say no, and I run through it with them.
For simplicity's sake, I leave my camera on Program AE mode, and if it's indoors or dark I pop up the flash myself, before I hand over the camera. That way, all I have to do is show them how to pre-focus and look through the viewfinder - no other settings or controls needed.
I've only had one or two try to look at the LCD instead of the viewfinder. But most look at the LCD after the shot to see the review, and are surprised when the pic doesn't appear (because I have auto review turned off to save battery life).