However, not everyone eats the same amounts and you also have to consider the bread and cheese you may place on the Sloppy Joes. For some, a quarter pound on a bun with cheese is enough. On the other hand some people will eat at least two of these Sloppy Joes sandwiches in a meal.
Before you consider using 5 lbs of hamburger for 20 people consider what other food you may be offering with the main dish. Are you going to have two sides, drinks, and appetizers since it sounds like a party? If you will have four or five dishes available as side dishes and appetizers, you can probably use the 5 lbs for 20 people.
Additionally, if you will have more than one entree, chances are not all people will choose Sloppy Joes. As you are no doubt aware, there are some individuals who do not like the concept of Sloppy Joes, so if you have 20 people attending or less you are probably perfectly fine using the 5 lbs.
If you are aware of your guests' eating habits and know they will be more inclined to eat half a pound of burger then that will reduce the amount to 10 people per 5 lbs of hamburger meat. Bear in mind too that with the sloppy part added in, you can extend the meat a little further because it is not just plain hamburger, but a sloppy liquid mess with hamburger meat. Thus, you might want to consider using four to five ounces as your measurement to determine if you can extend the meal out to more than 20 people.