Option 1 is to do sloppy joes or made-rites, much simpler.
If you want to grill 100 burgers, allow about 10 minutes per grill-full if the fire is perfect- you have to figure out how many grills-full you need to do. Then you start so that you are just done when you want to serve.
That's a nice type grill. Many have a warming shelf. You can put foil wrapped packs of done burgers there.
As they come off the grill, to use the roaster, you put them in the roaster which has been preheated for at least 20 minutes to just 200 degrees. Put foil between the layers. Keep TIGHTLY covered between additions.
If burgers are the only meat, allow 2 regular per guy, 1 1/2 per girl for teens; 1 1/2 per adult, 1 per little kid.
Do a box of Boca burgers or other veggie burgers for the non-meat eaters.
Sandwich fixings are discussed in the sandwich article.