Simple really.
MC wanted an item for the dungeon. A ring to help with ... affection.
The others decided / realized that he can under no circumstances get it so they accompanied him to stop him.
He then fell through a wall and landed on the bottom floor and is now making his way upwards.
I like how they're just named Bitch, Bitch 2, so on. It actually helps me because remembering every single character from each manga and novels I've read so far is so...😕😟😵😦😥😭😢😖😩😡😠😬