(There's a schematic on ipdb.org.)
When you try to add a 2nd player does the Credit Relay fire, and does the Score Motor turn? Assuming they do, let's look at the Coin Unit which should advance to add a 2nd player:

CF coin unit (resized).jpg

When you hit the credit button for the 2nd player, does the Total Play Meter advance? If it does, the problem is that the normally closed side of the Make/Break switch on the Reset Relay is open. If neither the Total Play Meter nor the Coin Unit advance for a 2nd player, the issue could be in one of the other three switches in red in the schematic.

/Mark

I'm not familiar with the Bally Player Up Unit but it looks like it steps up once for each player for each ball, then resets to player 1 for the start of the next ball. (The Gottlieb Player units on the other hand only step in one direction and skip past the non-playing players.) So here is the relevant piece of schematic for the Player Up Unit:
CF player up (resized).jpg
The right side shows that when the ball drains and the Outhole Relay fires, a pulse gets sent through a Make/Break switch (in red) on the Player Reset Relay to either the Step Up or Reset solenoid on the Player Up Unit. The Player Reset Relay on the left (in red) should fire when the Ball Count Unit advances.

I suspect that your pulse (generated when the ball drains) is mostly getting sent to the Player Up Unit Step Up solenoid, taking you to player 4 every time. That's probably because either
- the Make/Break switch on the Player Reset relay isn't adjusted properly or,
- the Player Reset Relay isn't firing when the Ball Count Unit fires, perhaps because the Ball Count Unit EOS switch isn't adjusted properly.

/Mark

Aha. That helps. Check this out:

CF ball count (resized).jpg

It looks like the Ball Count Step Up solenoid should fire when the Player Up Unit disc is on the last player and the Outhole relay fires. You say it works fine for a 1 player game. In that scenario, the pulse to the Ball Count Unit Step Up solenoid goes around the Player Up Unit disk, through the Gray-Red wire and through the Coin Unit Disk. For 2 and 3 players it misbehaves, which would require the pulse to go through the Player Up Disc and through the Coin Unit Disc.

The problem may be that the pulse is getting lost in either the Player Up Unit disc or the Coin Unit Disc. Since the Ball Count unit does eventually fire, it seems that the Player Up Unit disc is advancing and eventually connecting to the Ball Count Step Up solenoid by bypassing around the Coin Unit disc on the Orange-White wire. That makes me wonder about the Coin Unit disc (in red). It seems to work fine in the Player 1/reset position. But does it advance properly and make the connections for multiple players? If the wiper on that disc steps off the rivet for player 1 but never lands on the rivets for the other players, that could explain what you're seeing.

With the power off you should be able to put the Coin Unit disc in position for various numbers of players, then check for a very low resistance (not just continuity) between the Orange-White wire and the three wires on the opposite side (Gray-Red for 1 player, White-Brown for 2 players, and White-Yellow for 3 players).

Alternatively, you could start a 2 player game and put a jumper between the Orange-White wire and the White-Brown wire and see if that plays correctly.

/Mark

Same basic idea. Is there near zero resistance on the Player Up unit between the Grey-Red wire and the White-Brown wire for player 2 on a 2 player game? Or between the Grey-Red wire and the White-Yellow wire for player 3 in a 3 player game?

Taking a step back, you're trying to verify that there's a good connection between the 3D score motor switch (Grey-Red wire) and the Ball Count Step Up solenoid (Orange-White wire) in each of these cases:
CF ball count 2 (resized).jpg
- 1 player game, Coin Unit disk in position 1, Player Up Unit disk position doesn't matter
- 2 player game, Coin Unit disk in position 2, Player Up Unit disk in position 2
- 3 player game, Coin Unit disk in position 3, Player Up Unit disk in position 3
- 4 player game, Coin Unit disk position doesn't matter, Player Up Unit disk in position 4

These are the situations that should cause your Ball Count to advance but only the 1st one seems to work reliably. Checking the other three paths might reveal the problem.

/Mark