When you first get a Hideout at Master level 3, it is pretty small, and can only house 2 masters in it. In order to get more Masters at your Hideout, you need to upgrade it, and this is done by leveling up the master that is responsible for hosing your hideout.
At level 5, you can upgrade the hideout to host 4 masters.
At master level 7, you can do the final upgrade to the hideout, making it capable to host 7 masters.
Note that Leo and Navali does not count towards the amount of masters in the hideout, so you can fit all masters into a fully upgraded hideout.