Log out of all Intuit web sites in your browser. Accepting an invitation while logged in may cause it to fail or redirect to the wrong user ID.
When your client finishes inviting you, check your email and open the invitation.
Accept the Invitation
When you receive the invitation, the email subject will read the following:
[User] has invited you to use QuickBooks Online [Version].
Click the Accept Invitation link inside the email. It will open in your default browser.
Select the user ID for the firm associated with the new client.
If you already have QuickBooks Online Accountant, make sure to choose the user ID that you use to log into your firm. If you choose a user ID that does not have a firm, a new firm will be created under that login.
Enter your password and choose Accept Invite.
You'll be asked to sign back into QuickBooks Online. When you do, your new client will now be shown in your client list.
Now you can access your client's QuickBooks Online account as an Accountant.