The characters literally tell you why they join right in their dialogue. Farasat and Gavrielle are on a mission to rescue their brother from the mines; Madeleine is enjoying all the fighting; Louise stays purely out of loyalty to Madeleine (and actually will leave the group if Madeleine dies); Scarlet believes in the cause; Rebecca sees it as an opportunity to advance her career; Tremolo is in it for the loot; Harynx is an abolitionist; Harriet is on her own, parallel revenge quest; Teresa and Phoebe tag along in exchange for protection while exploring ancient crypts; etc. etc.

As for why the other characters listen to Emma: I dunno. Maybe because she wins them battle after battle after battle? By the time you get to the Battle with Ebon Raban, Emma's won 15 fights in a row. "Inexperienced" isn't really an accurate descriptor by that point.

That said, you're absolutely right in saying that Emma is dangerously naive: this is one of her character flaws. The story acknowledges this and explicitly brings it up as a plot point a few scenes on from where you are now.

Except for the brother/sister pair and maybe Scarlet, most of that is absurdly weak justification for joining this sort of quest. What you have in the game is perhaps enough for them to join in their initial introductory battle, but it is not nearly enough to keep them there after that. To put it into a real world parallel, imagine being mugged while on vacation in Mexico and then stumbling across a pair of runaway teens shortly afterward who want you to join their seemingly suicidal crusade against the local cartel. Never mind the fact that most of the people who join should be diametrically opposed to each other. You have the beginnings of an outline of a character blurb, nothing more.

And Emma isn't winning the battles, we as the players are. You as the developer need to do the work to show her actually directing the battles and taking charge from the beginning. is she really the one winning them? Why would anyone listen to her at the start? What does she do when they don't? You need to show that in the game; it cannot simply be assumed merely because her face is on the cover splash art.

I'm aware this is primarily a (quite well done) tactics game, and both the plot and strategy/party management layer can at best be described as barebones. However they are so lacking and poorly implemented here that they feel like hastily tacked on afterthoughts; it's a heavy detraction from the overall experience.

It's an SRPG; the "RP" part of that equation is you, the player, role-playing as Emma. Saying that you are winning the battles is only half correct: you are winning them as Emma.

And honestly, the character justifications here are no thinner than you see in most games of this genre. Pick any Fire Emblem game and I can rattle off characters who have no realistic reason for joining up. In Path of Radiance, you're a mercenary company that never pays anyone (except, hilariously, for the one character whose purpose isn't to fight). In Three Houses, characters straight-up defect from their home countries because they personally like you. Etc.

It's just a scène à faire in an SRPG like this. When there are dozens of unique characters, you can't expect every one of them to have deep, fleshed-out justifications for sticking around (and especially not in an SRPG that's developed by just one person).