Where and How to Unlock Chars
You can get Maltim Blanca very early just by going to Genoa and then exploring off the coast of Tunis.
Marma Cora in Veracruz by doing an expedition, heading west and getting a special event.
Garaba as a stowaway (I think when sailing into Oceania for the first time from South America).
Beatrice joins after the tulip quest line that starts in/around Amsterdam, I think you have to go there during certain months when the tulip auction and festival are held (November/February).
Lu Liyuan (Accountant) joins you after you encountered him once, followed him to Instanbul and found him in the Western Turkey expedition point.
Joyce de Fritz
Joyce de Fritz (Surveyor) in Amsterdam.
Shen Shi (Flexible) seems like a random encounter on the sea around China/Ming Empire.
Yuskin after he tells you he goes to Venice after you finished the quest involving him. Visit Venice and he joins you.