If your client accidentally logs out, the only way they can log back is through the Client Portal URL link. They need this link to log back in since they can’t use the same login page as you. Make sure to share this with them. This is the link from your client portal tab- not the Workflow tab.
Invitation URL link: The first link you shared with your client which allowed them to create an account to view all of your shared campaigns.
Client Portal Login URL Link: The link from your client portal tab which will allow your client to log back in to view the campaigns after they had logged out (see below):