"Our biggest problem now is cell phones."
I'm guilty about not always remembering to turn my ringer off and equally bad about remembering to put it back on again. When reminded however, I turn it off promptly -- once I find it anyway. Very few people have my cell number, I get more junk calls and calls for the previous owner than I do for myself but lately I've been getting the IRS or whatever call during my Tuesday small group.
Maybe not the nicest way to handle it but I remember one teacher who would stop dead and stare silently at the person until they turned off the phone/stopped talking. Only took a couple of times before everyone got the lesson, but some people learn faster than others! Maybe a handout given to everyone with general information such as the name of the group, the time, the address, etc., plus a paragraph on cell phone etiquette.
I understand that life happens but when you are in a group you are not just wasting 5 minutes of your time, but 5 minutes of the time for everyone who is there. That's half an hour for a group of 6.
If there is the potential for a real and true emergency, set your phone to vibrate and tell people to text you. Keep the phone handy and in easy reach to discretely look at the message. Notify your instructor/group in advance that "Jack had to take the cat to the vet, I might have to take a call" or whatever it may be. Someone wondering about what's for dinner is not an emergency.