You should have the option to Demand Height from them. Although most of they of the time they'll fight you first.
Those girls will join automatically after day 10. They are the other body types you can select. The girl in the harbour will be at the Pier always and that unlocks on Day 3.
They do run out of Mana essentially, so they can't cast anything else. I will probably add something in extra to mitigate that eventually.