How does licensing work with guest users?

Hello Andrii Vynokurov . 

Short Answer: There is no concept of "free seats for guests" on internal-facing communities. 

Longer Answer: For On-Premises licensing for employee engagement (internal-facing community), each employee accessing the system requires a named user-license.  The system has a license key that limits # of "registered users".  For example, if you have 10,000 named user-licenses, the 10,0001st user will not be able join/register.   A "guest user" concept does not exist although if you had parts of the community that did not require authentication you might be able to make it work.  But, most internal-facing communities are behind the firewall and requires authentication to even see the site. Most customers find the added expense to purchase the contractor licenses is worth not having to build in workarounds.  For SaaS deployments, the license is keyed similar to the On-Premises with user-license limits.

For On-Premises licensing for customer engagement (external-facing community), the licensing is unlimited users as we license the servers and domains - not per user.  For SaaS, it is unlimited users and the unit of measure for billing is page views/API calls consumed each month.