Is there any way to grow a party member or enemy beyond either sparring (party/potential party members and the Bind) and letting them use their native growth abilities on you, if you can tank them. Or more than the very limited amount that the bind does per submissive choice after beating. Barring those "natural" manners. Having to take the demon corruption to get incubus call. Which in current builds of the game is incredibly difficult to make non lethal in one blow even with keeping your limits fairly low. Plus the new combat system (I've not played since somewhere around 7.9 an age so it's lining up a combo isn't intuitive, yet at least.) makes stringing out a combo very difficult since combat moves forward automatically.
From what I can remember,
For Ferro, if you do his sex scene and submit to him each time, he'll gain a rank in a ability that makes him grow after you win any encounter.
And, for the imp, if you completely dominate him, you can grow him with essence when you check on him while he's your companion. If you fully submit to him, though he'll grow after you win or lose to him.
As for the other encounters, yeah, sadly that's the only way you can grow them.
Edit:
I forgot Ethrex, you can grow him if you lose against him when sparring with him while he's your partner.