人気者がそろっているというだけで、華やいだかんじになるというものだ。
Simply having popular people around makes for a cheerful feeling.
I think the ものだ used here is number 2) from this answer. Please correct me if I'm wrong. I was wondering what additional nuance という adds. Can it be omitted without changing the meaning?
I guess the same question can apply to the first half of the sentence (というだけで). Do the two というs pair up somehow?