Takanobu the Masterless's Skeleton Key Room doesn't have the option to Team Up. I'm pretty sure that must be an error, since all the other Skeleton Key Rooms have the option.
I was just in there today.... It's part of the whole instance, so to team up for that particular skeleton key boss you'd have to do the team up for the Hollow Mountain instance itself. Once your team is inside work your way through together until you reach the skeleton key door.
Alia Misthaven