If P1 and P2 have Pokémon Mysterious Dungeon Red and have a Game Link Cable, there is an additional feature. P1 sends the SOS Mail to P2. P2 rescues that person, then when sending the A-OK Mail, P2 can also send a helper.
A Helper is just a Pokémon that P2 has recruited. P2 sends that Pokémon to join with P1's Rescue Team. P1 will only have that extra Pokémon untill P1 completes that Dungeon. Then it automatically disappears. P2, who sent the helper, will still have the Pokémon in its Friend Area. It doesn't actually go away.