By this point in the story, John "The Savage" has brought the previously unpopular Bernard quite a bit of fame and positive attention. Before John brought Bernard back, Bernard was an unpopular, strange, unattractive, lonely nerd that not many people wanted to be around. He chose to do things by himself, copped an attitude of surly rebellion to mask his desire to fit in, and had a hard time getting the ladies or being the cool guy. However, when he brought John back, all of that changed. John was so unusual, such a novelty, and such a curiosity to people that all of a sudden, Bernard, the guy who brough back a savage, became the "it" boy. Everyone wanted to invite Bernard places, because that meant that they got a chance to see John also. They really just wanted to see a glimpse of this rare prehistoric creature, not hang out with Bernard, but, Bernard revels in the attention and fame, soaking it up and living it up while he can.
For this particular party, John doesn't come out because he has become severely disenchanted with the world he's been taken to. Before, he was curious about it, and held it up in an idolized light. When he gets here though, he realizes how fake it is, how people aren't truly happy, how people aren't intelligent, and he has serious moral issues with how things are run. He is tired of being poked at, questioned, studied, and looked at as some sort of historical relic plaything for a bunch of shallow people. Bernard has dragged him around enough, and he just wants to be alone. In his civilization, being alone wasn't uncommon, but here, there were always people around. Plus, the situation with his mother has really upset him also. So, he refuses to come out, but sulks in his room.
Bernard is furious, but has to save face for his party friends; he goes out and announces that John can't come, and tries to get the party going anyway. This is when his "friends" show that they don't really care for him at all, and leave. They utter insults at him on the way out, calling him a low-life, having gotten alcohol in his embryonic blood, and a fraud. Bernard is left alone, his ego-bubble burst completely, and frustrated at the role that John played in all of it. I hope that helps; good luck!