Sharing makeup is generally something you shouldn't do. Although, there are times where people do share makeup, which is completely understandable.
The biggest no-no of sharing makeup would be mascara. I'll tell you why. Everyone has bacteria on their eyelashes, so when you apply your own mascara, bacteria gets on the wand/bristles. Then when you put the wand back into the tube, aka a dark, moist spot, all of the bacteria multiplies and it gets a lot worse. So when you lend that mascara to a friend, and they pull out that wand, it has all the bacteria on it that was inside the mascara tube. That is why cosmetic counters offer mini mascara wands, so that when you try out a sample mascara, it stays sanitary. :D