This is a slang expression common in many regions of the former Soviet Union. For obvious reasons, in online gambling this word is not used in the same context.
Passenger – a player who substitutes his chips to someone else's bet.
Usually, we are talking about card games: poker, blackjack, and others. On roulette, chips standing on the same field are considered different bets.
In most casinos, this practice is not prohibited, although some institutions do not welcome it.
The joint amount at stake is considered by the institution as a single bet. The rules usually say that "welded" customers do not have the opportunity to make decisions during the game. They must obey the owner of the main bet. If he decides to make a pass, change cards, double the amount or perform any other actions, the passenger is obliged to do the same.
If the owner of the cards does not want to accept other people's chips on his box, he can forbid to substitute chips to his bet.
There are situations when excessive activity or shamelessness of passengers causes misunderstandings or leads to scandals.