damn. they skipped a lot in this chapter from vol2 ln or more like recontextualised the plot to have it acceptable to get skipped.
there's a whole storyline between Claude, Leona (martial club), and Teresea(student council general affairs).
Claude and Leona were exes and he tried getting back to her.
also in that vain Claude was inviting Teresea for dates back in vol1. she expressed that she liked her man serious and dedicated to just one and wouldn't mind a man if he mends his faulty ways(talking about Claude)
(this fucker Claude was already two timing them in vol1 I feel?)
Yujin and Sumire was waiting for Claude at entrance when Sara threw herself onto Yujin after seeing him. she asserted that they alone should explore together and leave Sumire alone. there's back and forth between the two like Sara first kiss with Yujin(because of partner contract back then) and Sumire saying that she had her first night with Yujin and that she knows his sleeping face (Sumire was getting defensive because before this Sumire and Teresea were having love talks, what is he in her life, how much did she get with Yujin and when she should confess, etc talks) after that critical hit Sara went away running defeated by Sumire.
Yujin was contemplating adding Leona because she is Sumire's friend as long as she and Claude won't have this awkward atmosphere between the two. btw Claude was helping the duo exploring the dungeons.
then sometime Leona was added, turning it into 4 member party. then after next dungeon sessions Teresea was added because Teresea saw Claude exploring the dungeon and would like to join them(at this point both girls don't know they were getting two-timed).
It was just a matter of time and sessions before what happened in the manga took place. both girls are out for blood of each others' because they are strong-willed girls