From Sega Retro
The Team Player, known as the Multiplayer or 4-Player Adapter (two versions exist) in Europe and Sega Tap (セガタップ) in Japan, is a peripheral released by Sega for the Sega Mega Drive, allowing for up to four players to plug Mega Drive controllers into the unit for use in four-player video games. It is one of several solutions to combat the lack of controller ports, with the 4 Way Play and J-Cart attempting to achieve similar results.
The Team Player contains two connector cables which must be plugged into the Mega Drive's controller ports (though earlier models only use one). This puts it at an advantage over the 4 Way Play as it does not assume both controller ports are the same distance apart in all Mega Drive compatible consoles (thus meaning it will work with the CDX and X'eye). It also contains a switch, which can specify which controller is mapped to the Mega Drive's first controller port.
However, the game Columns III need the first version of the adapter to be played by 5 players because the second port on the console is used by the fifth player.
Initially the peripheral was incompatible with games developed by Electronic Arts which were designed to use the 4 Way Play. This was fixed in a later release of the Team Player adapter. Earlier models can be identified with the codename MK-1654, with newer versions using MK-1647. The newer models also contain an extra setting, marked "EXTRA" which maps the first two ports (A and B) to the Mega Drive's first and second controller port.