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This question already has an answer here:

There was one time that I could "join" the defending Pokemons for a gym that is already my color (the same team).

But there was anther time I could not.

What is a rule for when I can join?

marked as duplicate by DJ Pirtu, Wrigglenite, scenia, galacticninja, Ave Aug 22 '16 at 14:32

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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Every Gym has a level, indicated by a certain amount of prestige. When a Gym is level 2 and there is only one trainer in the Gym, you can join that Gym.

But if there are 2 trainers in a Gym and the level is 2, then the Gym is full. This is the same up until level 10.

Training in a Gym raises prestige, and when the prestige is raised enough you level up the Gym and then you can join it.

More info

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There can be an empty slot for your pokemon when you visit a friendly gym. You can browse the defending pokemon by swiping left and right. If there is e.g. two dots and one crown (total of three spaces (crown is for the best pokemon also known as the gym leader)) and by swiping you see only two pokemon defending it, that means that there is one slot empty and you can place your pokemon there.

There are as many spaces for pokemon as the gym level is (there is space for three pokemon defending a lvl 3 gym). Your color (same team) can raise the prestige of the gym by training against own colour pokemon. Then the level will rise and more pokemon can defend it.

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A gym can hold pokemon based on its level, for example a level 3 gym can hold 3 pokemon and a level 5 gym can hold 5 pokemon. You can increase the level of a gym by increasing its prestige like training at the gym and winning those battles.

Perhaps that time when you could join a gym of your team, there were fewer pokemon in the gym than the gyms level.

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