The reputation of people can be found on the right of their username in the posts; the green bar. By hovering your mouse of it, a comment of how well they are doing is displayed.

If a post deserves it, you can give points for good work, or for a bas post, give negative points. You do this by clicking the 'scales' at the top right of the persons post.

The more rep points you have yourself, and the more posts you have made, the more rep points you can give & take.