If you're using Confluence as a knowledge base for Jira Service Management, you can choose to allow all active users and customers (that is, logged in users who do not have a Confluence license) to view specific spaces.
Note that this permission can only be turned on via Jira Service Management and will override all existing space permissions. This means that any logged in Confluence user will also be able to see the space (regardless of their group membership).
You can edit this permission at any time to revoke access to a space, but it can only be re-enabled from Jira Service Management.
Active users who don't hold a Confluence license have very limited access to Confluence. They can view pages, but can't like, comment, edit, view the dashboard, use the space directory, see user profiles or search your full site.
See Use Jira and Confluence together for more information about Jira Service Management integration.