Pinball, as most people understand it, and as I used to understand it, was just a single player game. It was totally unknown to me that nearly every pinball machine made since the 1970s allows between 1-4 players to play. Often, when I go to the arcade, I see a lot of people who happen to be unaware of the multiplayer aspect of pinball, which I believe adds a lot of fun to the game.
Nearly every pinball machine operates on some sort of credits system. You put in $0.75 in quarters and on the display you see a credit. If you want to play a multiplayer game of pinball, here is what you do.
1. Insert money until there are as many credits as the number of players you wish to play (up to 4)
2. Press the start button once, but DO NOT PLUNGE THE BALL TO START THE GAME
3. Press the start button again, this should add another player. This is usually indicated on the game's display by showing a second score of "00" away from the current player's score
4. Repeat step 3 up to 2 more times.
Now, if you have done this correctly, each player will get a chance to play a ball before the next player will go (unless any extra balls are earned). This keeps gameplay more interesting since not only are you competing against the number moving arbitrarily higher, but also your friend. To me, what makes pinball especially fun is the competitive aspect. Tournaments will keep the game alive, because while the abstract concept of beating a pinball machine is hard to understand with pinball, crushing your friend is way more immediate, and more fun.
Be sure not to press the start button too quickly when adding your friends to the game, or the game may not register it. Always check the display to see if it worked.