Two things can happen when accepting an invitation link to be a User in QuickBooks Online.
First, upon accepting the invitation link, the User will be asked for a User ID and a new password. This will be used upon logging in to QuickBooks Online.
Secondly, if the User accepts the link with an existing Intuit account, then the password should match the User ID.
A Security Key will be sent to the email used upon accepting the invitation link to verify the email.
Since the new User was asked to log in without creating the credentials, then they'll need to reset their password or recover their User ID. No worries, I have the perfect Community Article that walks you through the process of getting the User logged in successfully: Log In Issues for QuickBooks Online.