How do you meet another immortal?
Do you have to arrange it with the owner of the character first and come up with a story or can you just meet any immortal?
You write a story, and they join. So the person joining decides to meet. If you want to invite someone specific, aim the story at them (e.g. write how you are searching for them, or walking towards their home, etc.) and wait for them to join. The full game will probably have some kind of notification if you get mentioned in someone elses story.
Basic principle: You cannot play other peoples' characters.