There are shared prizes anytime you or a friend earns a medal or a badge. The requirements for each medal are on their respective games' pages -- just mouse over them to see what you have to do. Challenges are a bit trickier. There are two challenges every week, and you can also set a "personal challenge" from previous weekly challenges (but just one at a time).
You also earn tokens from playing the games but those rewards aren't shared. ^_^