On the sides of each Joy-Con, there are four lights that indicate players 1-4. On the grip there are eight lights, which makes sense because the two Joy-Cons hook up to it. However, according to the patent that was released a few weeks ago, the lights on the sides of the Joy-Cons correspond to tubes on the grip which carry the light when connected, because having a separate battery and/or light in the grip would be wasteful. Since it appears to be the case that the grip will just be a piece of plastic with no electronic parts, it's a smart move by Nintendo to keep the cost down, especially for individual Joy-Con grips!
Discussion started at here by lm_Just_Saiyan_