Saying that the angels are colluding to price fix buys into Michael's assertion that "together, the men in that room account for nearly 100% of all angel deals". That means that their "deals" are the commodity in question, and they are free to do as they wish. If it's their money, then it's hard to make the claim that their money is the market. There's certainly more money in the world than theirs.
You don't see a problem with this kind of artificial market manipulation? This is no longer a market; it short circuits true capitalism and only serves to siphon gains from the seller (in this case, the company's founders) to the buyer, who will turn around and effectively try to resell (or otherwise exit) the company for profit.
Everyone seems to be convinced that price fixing only applies to sellers. That's wrong. It firmly applies to both selling and buying. It's fundamentally about market manipulation; taking steps to undermine the economy of the system for direct personal gain. That kind of behavior destroys wealth and erodes confidence in the marketplace.
With Groupon nobody is asked to not buy or otherwise participate in the market unless done through Groupon. Instead, it's like a grocery store co-op; members pool their money to get more buying power. This doesn't mean they agree to refrain from shopping elsewhere, as that would be collusion/conspiring to produce a market effect. IANAL