User blog comment:Kaido King of the Beasts/Chapter 799: A Situation of Much Gravity/@comment-1595565-20150903165231/@comment-2226235-20150903204746

Based on the Mangastream translation, they were asking to be subordinate crews, not to join Luffy's crew. I think it's something like a Yakuza relationship since they mentioned the whole "parent-child" thing and Bartolomeo mentioned sharing sake cups.