Hello there tempsite,
Thank you for using Apple Support Communities!
We understand that you are having an issue adding your MacBook Pro to your list of devices. We are more than happy to help you out with this.
In order to do this, please be sure that you know your Apple ID and password. To do this, you can try logging into iCloud - Sign In. If you know your Apple ID and password you would need to navigate to the in the top left corner> System Preferences> and sign into your Apple ID with those same credentials. If you don't know your Apple ID and password, check out this article:
If you forgot your Apple ID password
If you forgot the password to the user on your MacBook Pro, check out this article:
Change or reset the password of a macOS user account
If you would like to erase everything on the device and reinstall the macOS, check out this article:
How to erase a disk for Mac
How to reinstall macOS from macOS Recovery
Please keep in mind that you will need your Apple ID and password in order to sign into your iCloud account during the initial setup after erasing the device, so please be sure you know that information or change it. Signing into your iCloud account will add the device back to your list of devices.
We hope this helps! Have a great day!