To add a device to your account, just sign in to your Premium account from the Hotspot Shield app on that device.
Learn how to sign in to your Hotspot Shield Premium account on the following platforms:
Max 5 devices per Premium account. You can have up to 5 devices linked to your Premium subscription/account. Once you've reached the 5 device limit, you will not be able to sign in from a new device until one of them is removed first. Learn how to remove a device from your account
In case you have a free account, you can try a Premium account for free here.