How many lbs per side should I have my friends bring for a crowd of 40-50 people?

Your menu is huge and I would cut it down some. Most people will eat potato salad, baked beans, tossed salad, mac n cheese and rolls and I would try and get people to bring those things. I would run those sides by my friends and try and get the ones who cook to make a recipe or double recipe of these things. A few big bowls of tossed salad with dressing would be much easier than a salad bar.