Employees can self-serve access their Blissbook account. They don't have to access Blissbook via a link sent to them and you don't have to trigger a special invitation email for them to click to get in. If the employee has an account in Blissbook (i.e. they are in the audience of a document), they can log in.
If an employee gets in touch with you regarding this error page, you can send them to either your custom domain or to your organization's blissbook.com domain at https://ORGANIZATION.blissbook.com (replace ORGANIZATION with your subdomain).
You can also tell them to click the "sign in" link on that error page.
In either case, they will be prompted to log in via SSO if your organization has it enabled, or they can enter their email address to trigger a new sign-in link to be sent to themselves.
If you want to go the extra mile and trigger an invitation for employees, you can do so on your Reporting or People page. Open the employee's detail card by clicking on their name, then go to Actions > 1-Click Reinvite/Reminder:
If you want to read more about how tokenized links in Blissbook work, including information on how long links are valid for, please read our help article on Authentication Tokens.