Not sure what they mean by "round robin matches" though, either it's a round robin or matches, but in any case Kasparov and the top three of the US Championships will participate. Kasparov vs Nakamura and So in blitz would be something :-)
Kasparov to play tournament

round robin matches may be either that you call a chess game "match" - or that within the round robin, every time two people meet, they play not one game, but, let's say, six.

He will not do to Naka what he did to Short... I predict Naka edging him out by the narrowest of margins, after some tough tactical fights in the Najdorf.
This could go very badly for Kasparov. He is playing blitz against a much younger man who is known for almost obsessive blitz play and is playing professionally every day. In fact I think Nakamura would win.
Kasparov's psychology is all about "great men" of chess. I think he would take it very seriously if he lost, especially badly. He had a good time stomping on Shorty last April and he should have left it at that for a long, long time.
Kasparov played (and won) a blitz minimatch against MVL in 2011, but he was just outside the top 20 then. This could be the first time Kasparov plays top ten opposition in some sort of at least semi serious competition since his retirement in 2005.

Kasparov has surprises in store for Naka. Opening knowledge and understanding, a sharp eye for the battle, bold, far-reaching sacrifices and great tactical acumen.
Even if he loses - so what? He's been in retirement for ten years and is fifty two...
It seems as if Norway Chess and the US Championships are played at the same time, but maybe Nakamura and So will not participate in Norway.

Who is this Naka guy? Is he facing Kasparov? He must be really good. I hope he doesn't get too traumatized.
One thing that could be worth noting, the three players kasparov plays will be fatigued to some extent after fighting for the championship, kasparov will be fresh. I wouldn't count ol kaspy out.
After Nakamura's comments on Kasparov, Nikitin, who had been on Kasparov's team back in the day, said: "I would like to see Nakamura play against the Kasparov of 1985-86, and see how much was left of him afterwards"
http://whychess.com/en/node/11083

I'm really looking forward to this! Kasparov's main weaknesses will be rust and lack of manual dexterity. Naka's main weaknesses will be defense. I'd love to see an extended match between Naka and Kasparov. If Caruana or Kamsky are also involved in a match-play tournament (I'm thinking of something like the World Championship 1948), we could be in for a real treat.
The fact that the blitz will take place over two days suggests that the matches will be substantial and not just one white, one black.

If Nakamura, Aronian, Carlsen, So, Grischuk, Kramnik, Giri and Caruana are not participating - I suppose Kasparov shouldn't have outstanding problems in his blitz brawls.

Why do you think Nakamura won't play, and why would Aronian, Carlsen, Grischuk, Kramnik (Kramnik? he's not a fearsome blitz demon anymore), Giri want to play in the US championship?
It would be cool to see Naka, So, Caruana and Kamsky fight it out for the US championship! That would be, by far, the strongest US Championship ever.
Even if it's "only" blitz it must be Kasparov's first round robin in quite a while, and he could face Nakamura in it:
http://www.saintlouischessclub.org/blog/2016-us-championships-press-release