You, Toriel, Asgore, Sans, Papyrus, Alphys, and Undyne are the first ones to go to the surface, just to make sure you can really get there,
but when you've established that there's a path to the surface, Sans volunteers to go put a few words in the right ears––or flames, in this case, you suspect––to get everyone moving.
By the time you're halfway back the way you came, you're being overtaken by the first monsters headed out of the underground.
They're so excited and moving so fast that you're a little worried you're going to be bowled over, but Undyne takes her job as the head of the Royal Guard very seriously.
She herds the rest of you into a tight knot and points you back towards the surface, since, as she puts it, that's where Asgore's subjects will need their king the most.
(She doesn't look at Toriel. You're pretty sure she just doesn't know what to do with her, but the whole "chewing out the king in front of third parties" thing has thrown her off a bit.
She might even be a little bit scared. Fair enough––you'd thought Toriel in guardian mode was scary, but angry ex-wife Toriel scares you in an entirely different way.)
Read the rest via the link in the description!