The two special groups in Confluence are:
All users who don't log in when they access Confluence are know as 'anonymous' users. By default, anonymous users don't have access to view or change any content in your Confluence instance, but Confluence admins can assign permissions to this group if it's required.
When a user is assigned more than one permission, the more powerful permission will prevail.
If you're using Confluence as a knowledge base for JIRA Service Desk, your JIRA Service Desk administrator 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.
These users have very limited access, and cannot be granted permissions in the same was as an individual or group. However, it's important to note that this permission overrides all existing space permissions, so any logged in Confluence user will also be able to see the space (regardless of their group membership). This is due to the way Confluence inherits permissions.