There will be no further permanent party members before the time skip beyond the current 12. Only temporary and for lmited areas. However, it's likely that more characters will be available as playable party members after the time skip.
Jessy's CG scene requires that her squad is level 3+, that you had the teasing CG scene with Vampire, and that the maid Louise is in the treasury.
Ailluin's CG scene starts in the barracks of the vanguards when her squad is level 3+, and after you unlocked the CG scene with Jessy.
You can use Ctrl+F to use for keywords in the walkthrough. Both of these scenes and how to get them are explained in it.