For 100 people you will probably need 2 roasters, unless you are making very small helpings.
Assuming you want to cook a roaster full- if it is less, just cook it in a covered pan in the roaster as if the roaster were any other oven-
this works for up to 6-8 pounds of rice. Do NOT fill the roaster fuller, it will boil over:
Measure out the rice and water into bowls.
Put the water and desired salt and seasonings in the roaster, bring to a boil with the roaster at 350 or above. Allow up to 20 minutes per gallon for this- it can take a while.
Stir in the rice and 3-4 tablespoons of vegetable oil, bring it back to a boil, turn the heat to 300 and clap on the lid.
Cook covered about 40 minutes, stirring once about 10 minutes in. KEEP IT COVERED.
When the rice is done, put a clean cotton dish towel over the top to keep moisture off the top layer, turn down to 170-180 and keep it covered until you fluff and serve.