What's a cheap meal I can make for $8 that will feed 4 people?

There's plenty... assuming you have basic pantry staples...oils, flour, spices etc..

You can make stir fry, fried rice, a quick chili/soup or pot pie, a chicken and rice casserole, etc. 

Rice, pasta, potatos, beans etc are all inexpensive and can make a good base.

If you can't go meatless.mBone-in/skin on chicken or eggs are your cheapest source of meat based protein. 

A bag of frozen mixed vegetables is often cost efficient and versatile in many dishes. 

2 boxes of hamburger helper and as much hamburger as you can get for what's left. Filling and delicious.

You could get a long loaf of French or Italian bread, slice it in half vertically, fill it with meat and cheese slices, lettuce and tomato and cut it into 8 pieces. Serve with some chips and pickles. 

A bag of rice and a few cans of black beans.

8 cans of chef Boyardee. They usually sell for a dollar a can. 

Maybe fried rice or stir fry if you have a bunch of unused veggies. 

Impossible.    You might serve your guests a bag of dogfood, and it would cost more.