Being active in Crowds earns you points. Your score is the sum of points you have received from answers you contribute (1 point per answer) and the likes that those answers receive (5 points per like). The more helpful and appreciated your answers are, the higher your score will be.
In short: The more helpful your reply, the higher your score will be
You can check the leaderboards, which can be filtered by category and period, to see how you compare with other Experts. Leaderboards are set to the UTC standard.
At the end of each season (which lasts 3 months) the top 5 users will receive a reward. We've not quite nailed down what that will be just yet but most likely a game or DLC of your choice as well as a special forum avatar (and possibly special icon!).
There is no limit to the number of times a person can place in the top 5 spots but please note that avatars and/or icon can only be rewarded once.