Hi there!
To locate a device that has been lost or misplaced with Find my iPhone (ipod touch, iPad or mac) you have to enter the Apple ID associated with it and the password, you can't "add" it as you'd have to do the before mentioned to see its location every time.
In the other hand you can use "find my friends" which is a different app which allows you to "add" your wife, other family members and friends and you can request to know their locations more frequently, then they'll get your request and after accepting it, you'll be able to share locations.