Is it possible for players to gain an advantage from colluding in blackjack?
In a loose sense, players can collaborate by signaling each other their hole card when the deal is one up one down. this allows far better optimization by almost doubling the known number of cards for the sharp who counts. Especially true if one or more goes for small bets and lots of draws.

The more cards are known to a sharp, the better he's able to account the odds. 

Note that signaling, if caught, is potentially grounds for removal from the facility and forfeiture of any chips.