Depends on the size of the burgers.... You can make 4 quarter pound burgers per pound, 5 pounds will give you enough for 16 burgers.
If I Have 15 People Eating Hamburgers How Much Ground Beef Do I Buy?
Is this a all day meeting/picnic/cook out or just for a meal.if all day I would go with a pound per person if they are all adults.also depends on if all the people coming to the meal eat hamburger/meat.it also depends if you are having sides with the meal maybe some ice cream or pie then I would say make it 3/4's of a pound per person.so either 15 pounds of hamburger or 12 pounds.you could always buy 15 in case and if you don't use it then save it for meatloaf or a burger later.blah blah blah.