Basically once you rank up to level 5 you can to use boosts for your FC, it used to be 5 but now SE added stuff so now the Free Companies are up to lvl8.
Leveling your FC is more of a group effort as doing fates together, dungeons, etc. get you fc points faster. Myself i have been grinding in fates in a effort to level up.
As for deposit it is best set for Full but access only by leader or your second in command