Like I described in the summary, Amity ran away from home at 13, which is a year before Luz comes, since they're both 14 at that time. 

Amity quite literally ran into Eda when she was running through tents trying not to be seen. Amity recognized her from her posters and she thought that she would be the last person anyone would expect her to be with, so she hoped the Owl Lady wouldn't try to kill her and asked for her to take her in, while promising to do whatever she needs. 

Eda who didn't really care, but could see how desperate Amity was, she decided to take her in and allow her to work for her.

(Amity and Eda aren't close the way Luz and Eda are, before Luz joins they still take care of each other, but like they don't do a lot of stuff together besides work.)

The story will start in the first episode when Luz arrives.

If you have any questions, don't be afraid to ask!