If Yamato joins the Straw Hats is he part of which of these groups?

Group 1 - The Cowards

Nami, Chopper, Usopp

Group 2 - The Adults

Robin, Franky, Brook, Jimbei,

Group 3 - The Dumbasses

Luffy, Zoro, Sanji,

I would say adult since Yamato is 28, but he's quite the dumbass. Dumbass it is.