Rounding out characters
Question: I have more than one main character because it's like a group of friends but I can only seem to think of things to happen to one character. How can I work on rounding out the other character because right now they're just there and if anything happens to them it's because the one main character has something to do with it?
You might start by considering why theses characters are there in the first place. What skills, drives, and perspectives do they bring to the table? Take a look at this article as a starting point to differentiate them...
Another tactic is to try writing from their point of view. For each character, you could write about the best/worst thing that ever happened to them. You could write letters in which each character explains why they are participating in the story - what it means to them personally. You could write about their dreams and ambitions, their fears and worries, how they feel about the other characters, who the important people in their lives are etc.
The more you come to understand your characters, the more they will acquire minds and wills of their own. They can then start to surprise you by doing and saying things that affect the story in ways you haven't yet considered.