A scraper is a person who tries to muscle in on action that they are not involved in. It could be a picture, whereby somebody shoves themselves into a photo without intention by the photographer, or somebody who tries to join in enthusiastically on a conversation which they know nothing about.
Person 1: 'Did you see Jane's skirt the other day!'
Person 2: 'yeah atrocious'
Scraper: 'omg yeah' (scraper did not see Jane's skirt)