In Chapter 11 of both Eirika and Ephraim's routes, L'Arachel can be recruited by talking to her with Eirika or Ephraim, respectively. Once recruited, she can talk to Dozla to recruit him. In addition, L'Arachel will join the player on the adventure as long as she is alive at the end of the chapter. Of course, I cannot think of a reason why you would want to wait since you can just go ahead and talk to her using the main lord to recruit her right as soon as possible. Also note that if one of your units within her movement range is in need of healing, she might use her turn to move closer and heal them. This could make it a little easier to reach her with Eirika or Ephraim, if you find that it is tricky to reach her without her moving closer on her own.