First, you must understand the purpose of parties. First, as Francisco d'Anconia told Hank Rearden,
"Mr. Rearden, you do not know these people's way of doing business or how they interpret your presence here. In your code, but not in theirs, accepting a man's hospitality is a token of good will, declaration that you and your host stand on terms of a civilized relationship. Don't give them that kind of sanction."
In fact, Hank's wife had quite bluntly told Jim Taggart that her husband's presence at the wedding was evidence that she could "deliver" him whenever she wanted. She also pointed out:
"... your guests are quite impressed. I can practically hear them thinking all over the room. Most of them are thinking: 'If he has to seek terms with Jim Taggart, we'd better toe the line.' And a few are thinking: 'If he'safraid, we'll get away with much more.' This is as you want it, of course - and I wouldn't think of spoiling your triumph - but you and I are the only ones who know that you didn't achieve it single-handed."
The text makes it quite clear here (and elsewhere) that a large part of parties is to establish and reinforce the social "pecking order," showing who has influence over who and who. The fact that people thought that Jim Taggart was able to force Hank Rearden into attending the party meant that most people assumed that Hank Rearden was afraid of Jim Taggart for some reason.
The phrase "in furtive informality" is important here as well. In a sense, this likely helped them feel less anxious about the import and impact of what they were doing.