These are puzzles that require two or more players to co-operate together in some way in order to solve them. The puzzles should always reward all of the players who successfully took part in solving the puzzle, otherwise there is no incentive for players to help each other. Some multi-user puzzles may require that one of the players play a bigger part (or perhaps take a greater risk) than the others, and this player should receive an additional reward over an above the other participants.
Using the combined strength of two or more players to lift or move a heavy object. Using the combined weight of two or more players to depress a lever which then triggers some other action. Allowing one player to lift another to reach something that is visible but just out of reach. A monster that is too strong to be defeated by just one player. Access to something/somewhere is only possible while another player performs an action (usually in another location).