You do not need to add the device in Web panel if you already added in the mobile app. It will be synchronized automatically to your account. All you need to do is login to the web portal with same email/password that you used for mobile app login and go to My Devices.
1. One device can be added only in to one account (email/password)
2. Same device can be controlled from multiple phones and web panel if user login with same email/password.
Hope it answers your query, If you need further hellp please post your query call our support number from App->About screen.