A gym is where most of the action that comes in Pokémon GO takes place, here players can take control of gyms by lowering the prestige of a gym that is not claimed by their team. They can also claim a gym that is colored grey, for their own team.
How to join a team Edit
After reaching player level 5, and tapping on any gym from any distance, you will be introduced to teams by Professor Willow, and you will be asked to join one of the three teams. The three teams that are available to join in Pokémon GO are Instinct, Mystic and Valor, also known as Yellow, Blue and Red, respectively.
How to join or claim a gym Edit
A player can join a gym by submitting a Pokémon to a gym that is owned by their own team, or claim an unclaimed gym. There must be an open slot available at the gym that the player wants to join, however. The slots open up as the gym levels up, leveling a team requires more prestige to be earned. The amount of players that can join a team, directly correlate with the gym's level, for example, if a team is level 1, the gym can only house one player's Pokémon, if the gym is level two, it can house one Pokémon from two different player.