As defined by the IRS

Collusion - Cooperation between two or more persons to defraud a third party.

And in Mike Arrington's case, collusion is more specifically referring to price fixing.  Mike has laid claim to the notion that angel investors have been collaborating to fix prices on startup company valuations.

"One source has also said that there is a wiki of some sort that the group has that explicitly talks about how the group should act as one to keep deal valuations down."

In this case of course, deal valuations are the "prices" that investors pay for stakes in compannies.

And price fixing can be prosecuted under the US Legal Code.
Title 15 (Commerce and Trade)
Chapter 1 (Monopolies and Combinations in Restraint of Trade)
§ 1 (Trusts, etc., in restraint of trade illegal; penalty).

Dave McClure, who actually was invited to these invite-only congregations of the biggest ballers in angel investing has his own take on the matter.

So does, Fred Wilson who is a venture capitalist that tends to do more early stage deals than others, and thus interacts more frequently with angel investors.

Is collusion really taking place?  Don't ask me, buddy.  Who's to say I wasn't one of the select angels??

Sidenote: shouldn't the US Government make the entire legal code available online?  If you know where it is, please direct me, as I could not find it and had to settle for Cornell's copy.  It never made sense to me to hold citizens accountable for laws which they are not fully aware of and do not have easy access to.  Isn't that like playing games on house rules, where the host jumps up every 3 minutes clamoring about how that was an illegal move and doesn't give you a redo?  ANNOYING.