Yes, you can get multiple people onto your ship, along with a few cats. In the case of the girl in Na-Nal, she first says she's playing hide-and-seek with her dad. Her dad dies during the attack on Na-Nal by Kooluk, but the girl goes on thinking her dad is still hiding somewhere. You can then offer her to come on the ship.
In that case there's a requirement for inviting her or a cut-off point that results in her simply disappearing like the OP said.
By talking to her she simply said something about having played for a long time but still couldn't find her dad and after that she disappears if you leave the area and is nowhere to be found. No prompt for the invitation either.