I wish to add a client who accidentily originally registered as GB quickbooks

What is GB?

I'm here to provide some information and ask a few questions to better understand your needs, ludovico.

What challenges are you encountering when adding clients? Is your client using a different region of QuickBooks?  While waiting your response, please know QuickBooks accountants are able to accept invitations to clients within the same region. 

If that's the situation, your clients can send you an invitation as a Company Administrator to their account, giving you access to your file. Here are the steps to help you through this process:

1. Go to the Gear icon and select Manage users.
2. Select Add user. Enter the user’s name and email address.
3. Select the Roles drop-down arrow, then select the role you want to assign to the user.
4. Select Send invitation.
    

For more information, you can refer to this article: Adding and handling users in QBO.

Moreover, you can look into this helpful resource: Understanding user roles and access rights in QuickBooks Online.

Please let me know if you have more concerns about managing users in QuickBooks.