[quote=@Squashedquatch]
Lu, does this mean we are now in the under, or that you are in the under, and that we have to figure out how to get there to continue the story? 
[/quote]

In the course of my post, Purple Team boards the crane platform, descends, then falls into the Under. In order for the Adventurer to accompany them, you'll need to give him a reason to tag along. Being healed by a Purple Team member after the fight in front of Robbers' Barge will probably be the catalyst, since he just received the kindness of strong strangers for no real reason. He might feel indebted to Purple Team, or otherwise interested, be that in treasure hunting and adventure with them or maybe interest in their power/weaponry (either out of curiosity or perhaps a desire to possess it himself). As a result you should have him follow along and probably ask to ride down with them, and which point the Adventurer is involved in what's happening.