Presumably Ryuu but that's probably going to be it, if anyone joins in this volume that is (maybe Nina but that's just a head canon thought).

I think Nina would join at the end of the arc, if she did at all, so Volume 20 or 21

If HF gets any members from FF, then they'll also be joining in this book.

I had thought that Nina would be too young to leave the school district but she is the same age as Lefiya was when she joined Loki so Nina could join if she develops a very good feeling with Bell and company.

On the other hand, Bell's SS going down to the 37th floor occurs after DM19 and at that time no Freya executive had joined, which makes sense, in the hypothetical case that some of them were to join, it should be when the family Hestia plans to venture into expeditions that require a lot of power, not before then because, what role could they play in a mini family that doesn't venture very deep?

I think if someone from FF will join in the near future it will only be non-combatants like Heith and/or Horn since the role of fighters is covered by Bell and Ryuu for now.

@Leinad91 On the contrary, Nina is older. Lefiya joined LF when she was eleven. Eina and Misha both graduated when they were 14. Nina is actually about the average age to graduate and begin seeking employment in some fashion (because Oomori has all these characters run off into the adult world at absurdly young ages).