make somebody up means apply cosmetics to (e.g. My sisters made me up for my graduation party.)
make something up means invent something that is not true (e.g. What Kevin told you about himself wasn’t true. He made it all up.)
make up means forgive each other (e.g. We were angry last night, but we made up at breakfast.)
make up something/ be made up of something means invent something that is not true (e.g. Children under 16 made up half the population of the city.)
mix up people/ things, get people/ things mixed up means you think one is the other (e.g. The two brothers look very similar. Many people mix them up. (or ~ get them mixed up))