Even if it was a bad comparison, I don't get the logic behind thinking that someone will join solely because Luffy invited them. You're still very clearly ignoring all the reasons that Loki can't join the crew, including the fact that:
He's far too important to be Luffy's subordinate, given that he's the new God of War as well as the heir to the throne of Elbaf (it's not even remotely implied the former War God joined Joyboy/Nika's crew either).
He's far too big to fit on the ship
He's far too strong to be in the crew
Ragnir is also far too big to fit on the ship
there bonnie if she cabin girl- she can imagine loki normal size
there lilith-vega punk brain, to find a solution to shrink loki down with some device/invention.
there also chopper as a big if, for his medicine that make him a giant... so wonder if nidhogg ate pill, can be shrink modifier to his dragon form. letting him by pass the giant growth.
there the owl, shown to enlarge objects, not confirmed if he can shrink size down to.. but possible.
there actually plot potential to solve loki size issue in the above, and i rather wait till end elbaf to see this.. because lilith for example is a plot device that can be used to shrink loki later if that why oda brought her to elbaf instead of her escaping with the other vega punks.