Are you sure the girl was uncomfortable with the fact that a co-worker had his arm on her and not that you were starting at her and analyzing it? The former is pretty normal as far as friendships go.
FWIW, it wasn't interactions between co-workers who know each other. The outing was a dozen or so male employees of a tech company who were later joined by four employees of a recruiting company who provides us with candidates. The only female in attendance is an employee of the recruiting company. She was the recipient of all the attention in all 3 cases. I believe only 1 of the 3 guys who crossed a line had ever met her before.
Potentially also of note is that none of the male recruiters said anything. It may be that they generate a lot of business from us and weren't willing to say anything for that reason. She definitely seemed to be the junior member of the team.