The main information area shows all referees who have been assigned abstracts. Referees with no assignments are not listed - so the list will at first be blank. The list shows the number of abstracts assigned and those that have so far been graded.
Abstracts can be refereed either by referees or committee members. In both cases you must explicitly assign abstracts to them.
The difference between referees and committee members is that committee members also have access to the Committee screen which gives an overview of the entire refereeing process.