Agreed, take charge!
Whenever there's group shots and everyone else is taking pictures causing people to look in all directions I usually wait a few seconds for the "snapshotters" to get their picture and then with a strong & clear voice (and a smile) I say: "Ok guys, everyone look at me please!" (If it's a fun crowd I'll say something that makes them all smile or laugh if possible). It hasn't failed me yet. People will focus on you for a few seconds before they start drifting again. Repeat as required.
If it's posed group shots where I'm posing them I also find that moving onlookers to the side helps, that way no one "catches the eyes" of those standing behind me. People in the direct line of sight of the group will always cause someone to look at them instead of the camera (especially kids. The attention span for boys is about zero and for girls maybe a second).
There's a thousand ways to do it, but take charge and you'll get the picture you want.