On the myDevices PC/Mac dashboard, log in and click the ‘Add Device’ button in the upper right. Then, select the tab for the type of device invite you would like to send. Be sure to mark it as ‘not my Device’ if you want to manage the device for them.
Your invite recipient will then receive an e-mail invite that has steps for them to install myDevices and associate their device with your account.
Invites can also be sent from the iOS/Android mobile app by sliding out the device list (swipe from the left to the right while in the myDevices app) and clicking the ‘Add Device’ button at the bottom.