One device can only be added in one account. If the device was added in other account before, you will get error"Device has been registered".

Go to NVR Menu--Network--Basic--P2P, click Unbind current account or Delete on the botton of the page to unbind the old account. Then add device on your phone again.

If other users need to view video on their phone, please follow the video below to share device to them.