Where is the add pokemon button to a friendly gym with only one pokemon in it?

I'm at a friendly gym and would like to add a pokemon, but the add pokemon button is not showing. There is only one pokemon here. Where is the add pokemon button? I'm at a high enough level but I have not been able to add a pokemon to any gym.

Solution 1:

The most likely cause of this is that all of the gym's slots are full, which means you will not be able to add another Pokémon until the gym is upgraded. Gyms can hold a number of Pokémon equal to the gym's level, so the gym in question is likely only level one, and its slot is already full. To increase a gym's level, you must win friendly "battles" (training) against the gym using only one Pokémon (as opposed to the usual six). Every time you beat a Pokémon at that gym, its prestige will raise, eventually leveling it up and adding another slot. At this point, you should be able to add a new Pokémon to the gym.

As Riley mentioned, the button will appear in the bottom left corner of the gym when there is an empty slot, and any Pokémon that you wish to place in that gym must be full health.

Solution 2:

The button is in the lower left corner, however your Pokemon needs to be at full hp.