Unity - Scripting API:

public static function AddPlayer(readyConn: Networking.NetworkConnection, playerControllerId: short): bool;

public static bool AddPlayer(Networking.NetworkConnection readyConn, short playerControllerId);

public static function AddPlayer(playerControllerId: short): bool;

public static bool AddPlayer(short playerControllerId);

Parameters

readyConn	The connection to become ready for this client.
playerControllerId	The local player ID number .

bool True if player was added.

This is the local player ID for the player, for example like which controller a player is using. This is not the global overall player number.