Most of girls joins you after finishing your movement phase in a tile of certain biome.
Cami - plains (green). Zaria - beach (sand). Talia - aquatic (blue). Lia - forest (dark green with tree). Kinwa - swamp (dark green).
Oriana joins you after you free her from her coffin by exploding it with tnt in a drilling minigame. I could find coffin only in mountain (white) area.