When you participate in competitions, the leaderboard is formed out of competitors at a similar level. They are randomly selected, so you and your friends might be on different leaderboards.
You can tap the event icon on the left side of the screen to check your leaderboard position.
All the participants receive a participation reward, while the top 10 players win special prizes. If several players finish with the same result, they'll finish in the same position and get the same reward. The competition results are calculated within an hour upon the end of the event.
IMPORTANT NOTE: If you finish in the top but miss the result announcement, you'll be able to collect your reward once you're reconnected to the internet.