Let's say Loki said "Yes, i'm a Big Fan, let me Join"
Will They just Sign him up, invite him to Mary Geoise, explain his Job & call it a day?
This whole Magic Stuff is just Science in Disguise, Celestial Dragons Outfit make them look like Scientists, Leader of Holy Knights becoming God of Science, Saturn (Previous God of Science) was the one who Summoned other Gorosei, WG being Main Villains of Island of Science Arc, with Pacifista being New Smile Army & Seraphim being New Tobbi Roppo, WG replacing Ancient Kingdom who were all about Science, Imu using a Scientific Weapon to establish his Dominance ... etc
It's clear to me that WG is all about "Science, Bitch!",
So i believe that it's mostly about Clones, if Loki accepted to become a Holy Knight, They'd make a Clone of him,
Which would explain Holy Knight Shanks, meaning Shanks was asked before & apparently said Yes,
Gorosei are the same imo, They are Clones of other People, that's why We have Ghandi who doesn't fit with Europe Theme of Mary Geoise,
Just like Pacifista are also Clones, and Warlords were Cloned into Seraphim ... etc
Seeing how Loki was asked to Join + We have a Shanks Clone + Gorosei were implied to have Shanks Like Presence, it's pretty clear to me that WG only asks those with Shanks Level to be Recruited & They need to be CoC Users
Which begs the question of who was asked before,
Roger? Kaidou? Big Mom? Xebec? Rayleigh? Garp? Sengoku? WB? Dragon? Mihawk?