Perhaps the guys at the party are just as unhappy with their new lives as other newborns , but have somehow also clued in to the "alternative", and have learned not to rock to boat.
..And yeah, the old man makes it pretty clear that the organization is in a constant state of harvest/reharvest to keep afloat

Yeah, I thought it was pretty clearly a road test of his conditioning and the stability of his new personality, with significant numbers of the guests being either staff of the Company or Reborns there on assignment. Nora and the "two-faced" man from Harvard were both working pretty hard to manage (and observe) his behaviour. And, of course, John, his housekeeper.
To me it was pretty clear that was the moment he was flagged by the Company as a likely candidate for recycling. And that started me wondering whether any of the clients actually made a go of it, or whether it was just a giant Ponzi scheme, where clients were milked of their assets for a "second chance" they had no possibility of succeeding with, and then were harvested as raw materials to set up the next client for milking.
So then I wondered if the party was in fact intended to plant the seed in his mind that he had botched it, and start him moving down the path towards wanting to "start again, all over".
You might very well think that. I couldn't possibly comment.

I just thought it was an attempt to set him up with new art clients, so he would succeed at his new career.

To me, the company having him go to a boring cocktail party was a way of "testing" his ability to move on, even (and moreover) on known grounds. If he couldn't resist bringing back old memories and habits, it meant he wasn't ready for a new life. That's why all guests are reborns : the Company knew he was at great risk, and that the failure rate was high in such an environment. Whereas in the grape party, it was more obvious he would just loosen up to a "new him".

Again, what were they thinking?? Introducing him to other reborns who look like they all hate their new existence and are as boring as the old bankers/accountants they used to be?..there's a recipe for disaster.
If they wanted him to meet other reborns (which is a bad idea as it only gets him thinking about his past life) at least introduce him to ones who are HAPPY about their new lives
The smart idea was to keep him with Nora and the hippie crowd so he could start enjoying his new bohemian life.

SPOILERS
At the cocktail he would meet the other "reborns". Unfortunately he gets so drunk he starts talking about his past with the wrong people.
Fear is the mother of morality.

good point
When there's no more room in hell, The dead will walk the earth