When is a sub legal?

[Bellbuoy]

Not to hijack the thread, but this happened in our game this week, 4.4 seconds left in the game, Team A leads Team B by 3 points. Team B calls a timeout. At the the conclusion of the timeout Team B sends a player onto the court who has never been in the game and has not reported at the scorer's table. Does s/he become a legal substituion when the ball is inbounded?