Hello there.
It's our pleasure to help you to sort this out by assisting your needs here in QuickBooks. Let's work this out together on signing back to the invitation from your client.
If you previously successfully accepted your client's invite, you'll want to login into this URL: qbo.intuit.com. From there, enter your login credentials, like your email address and password.
However, if you haven't accepted the invite successfully, I suggest asking your client to resubmit the invitation or use another email address for the invitation. This way, you can get into your client's account. Your client will have to follow the steps written in this article: Resend an invitation email section.
In addition, you can check this article to learn how to add, manage, or delete user profiles in QuickBooks Online: Add and manage users in QuickBooks Online. Please know that this is a US-based article but the same principles can be applied.
Please leave a comment below if you have further questions related to QuickBooks Online. We're always here to help you resolve your concerns. Stay safe.
