Robert Kraft Billionaire Bro Down with Celtics Owner

Here's a couple of VERY rich guys hanging out -- Patriots owner Robert Kraft taking in the Boston Celtics game Monday night with team owner Wyc Grousbeck.

Kraft and Grousbeck have been hanging out a lot lately -- they were also together at the Celtics game Saturday night with another member of the Boston Billionaire's Club ... Red Sox owner John Henry.

The Celtics beat the 76ers on Monday in Game 1 of the Eastern Conference Semifinals -- Kraft and Grousbeck were high-fiving on the court during the end of the game.

So, who's got the biggest bank between the three? According to Forbes, Kraft's worth $6.2 billion and John Henry's valued at $2.5 billion ... but Grousbeck's net worth isn't listed.

With the Celtics valued at more than $2 billion, he ain't poor either.