You can recruit Ivy by finding the voucher which recruits her. Her recruitment quest should be self explanetory if you've played Fallout before. After recruitment Ivy will count both as follower and a romantic partner, yet she will see you as only a customer until her affinity for you rises. 

You can dismiss Ivy by speaking to her. Once dismissed there are two options available:

- Go home
Ivy will return to her home location, or a workshop of choice.