Last year they also stayed at Riverside as well as All Stars - I was told there were over 1,400 of them at Riverside. The only time we saw them was in the Food Court, where they were no real problem. They did, however, overtake the pool and hot tubs so you might want to look into where they are staying if that is an issue for you.
They do affect attendance in that they all go to the same park at the same time. We noticed that they followed the early entry park schedule. Therefore, if you go to a park other than the one that has early entry you probably won't even see them.