The Player and Person of Interest take place in the same universe.
Harold been screwing with Reese the entire time, because he's much easier to manipulate than Alex is.- Alternatively, Ada is a another counterpart to The Machine and Samaritan.
Mr Johnson is a former Player.
He has the right skill set for the job. Intelligent, cunning and physically fit. Wouldn't be a surprise that all Pit bosses in the game are former Players that end up taking the role for one reason or another.
- Confirmed. A contact in "The Norseman" makes a statement that Johnson was a former Player and a leader board called up by Cassandra shows that Johnson is considered to be one of the greatest Players ever. He later confirms that all Pit Bosses are indeed former Players.
Agent Nolan's family contains, or all are, Gamblers
She comes from a noticeably rich family. She has a personal and intense interest in finding out the secrets of the Game. Surely she might have some familial ties to a Gambler, or multiple ones.