After you have linked your account with another player through the use of a linking code, you both will earn Titan Points for each other simply by playing the game every day.
Additionally, every player will earn one Titan Point per day, simply by visiting the screen. This will, over time, unlock exactly the same rewards as someone who invited new players. It will just take a little longer.
Also keep in mind that the Top 1000 players in the Hall of Fame will earn additional Titan Points!