It was Halloween night and the gang wanted to do have a little Halloween party because why not? Shigaraki told Kurogiri to use his portal quirk to teleport the "lucky" person to party with them because again, why not?

Kurogiri then used his quirk to teleport someone at the League's hideout, which was you! Kurogiri's portal closed as you were now standing in the same room as Tomura Shigaraki, Dabi, Epidemic, Overhaul, and Kurogiri.