This is a classic Lord of Destruction feature that is not known to all players.
During a single player game or tcp/ip or open battle.net multiplayer game, press Enter and type the following:
where X is a number from 1 to 8. Press Enter again. The game will confirm your action.
Monsters' Hit Points
Each monster's Hit Points are multiplied by the number of players in the game according to a formula: Hit Points = X * (n + 1) / 2
where X = base Hit Points n = # of players in game
Assorted in a table it looks like this:
With increasing player settings the amount of Experience gained also increases according to a formula:
The Players X Command
In general the Players X command simulates an amout of players in the game equal X, although less players are actually in game. X cannot be lower than the actual number of players in game. For example, if you're playing with 2 friends a multiplayer game via TCP/IP, you can only choose a number equal or greater than 3. All other commands will be ignored.
The Players X Command only simulates unpartied players. (Important for drop mechanics as explained above)
Note that the Players X command doesn't affect monsters that have already spawned by the time you type the command. To make sure every monster is affected, typing the command while still in town is recommended. Players X does however affect all clickable items (i.e. logs, chests, urns etc.) immediately. In theory one could clear the whole area on p1, then set the player settings to 8 for maximised drops (i.e. minimised NoDrop) and pop all chests, urns etc. only then.