I will try to help you better understand how everything is tied together. First of all, the hub doesn't require an account or the hub to be "registered".
The registration is a function of the cloud services that Hubitat provides to enable users access to additional features such as logging into my.hubitat.com, using the Hubitat Mobile apps, subscribing a hub to Hub Protect, using the new migration tool that was released with the C-8 hub. Registration is also required to use third party cloud services such as Amazon Alexa, Google Home, etc. There are some cloud services that do not require a registration. These services include updating the hub, using remote access to Dashboards and Maker API.
The process of registering a hub is completed during the initial setup of a hub. During the onboarding process, users are prompted to create an account at my.hubitat.com. The email address that is first used becomes the admin account for the hub. This account is then linked to your local hub.
Managing an admin account (the account that a hub is registered with) is done from my.hubitat.com. You can access Registered Hubs on my.hubitat.com from your local hub by navigating to Settings >> Hub Details. Alternatively you can go directly to my.hubitat.com and on the main page (My Account) you'd need to select "Registered Hubs" which will take you to the same page that you can access from your Hub Details. If you created multiple accounts for different emails, only the first account you've used will display the registered hubs. Additional accounts would not be able to "see" the hubs on my.hubitat.com.
In the event that the initial account was not the one you meant to use with the hub, or if you ever need to replace the admin account, you can only do so from my.hubitat.com by following the instructions in this post: Replace Admin from my.hubitat.com.
If you need to reset the password for the admin account, a user can do so from the main page on my.hubitat.com by following the "Forgot Password?" link.
If for some reason a user is unable to reset the password, then a warranty case should be created by visiting below page. (since you sent me a private message you don't have to create a case, right now - I will follow up with you in private)