How to add someone on iMessage?

Hello Julie, open the Contacts app, tap the + (plus), fill in the details of the contact whose email address is linked to the Apple account, and finish by tapping OK.
There’s nothing else to do.
iMessage only works between people who have an iPhone, iPad, or iPod.
Your correspondent must also have enabled the iMessage option on their device.
If you also have an iPhone with at least iOS 9, you can set up the iPhone and iPad together, and in that case, you can send iMessages and regular SMS to any contact, whether they have an iPhone or not (the regular SMS; written on the iPad; are routed through the iPhone to be sent).

Fred.ml