According to Andriasang:
The site has information about the game's NESiCA IC card support and Players Guild features. Via the P4U Players Guild site, you can edit your profile, buy character colors and other goodies using the P$ (Persona Dollars) that you earn while playing the game, view your ranking, search for other players, and get the latest event information.
A Q&A at the site offers further details on the NESiCA and Players Guild features for those who can read Japanese.
Along with this, Atlus' official site has been updated with a four minute tutorial video: