A Final Draft 13 Site License is a license for Final Draft 13 that can be used by three or more members of an organization whether they are in a studio/production company or an educational/nonprofit group.
Instead of a single-user license, which is purchased and used by an individual, a site license is usually purchased by a production company, school or other entity with need of multiple licenses of Final Draft.
A site license purchase involves buying a number of "seats," which is the number of people able to use Final Draft 13 via your license.
When you buy a site license, you will be asked to make an account with an email and password. This is the email you'll use as the "administrator" of the license. You will be sent the instructions to sign into a dashboard where you can manage who else has access to the license, or you can follow the link here: https://licensing.finaldraft.com/orgadmin
From that dashboard, you send an invite to whomever you wish to have access to your Final Draft 13 license. They then accept the invite, and will be asked to make their own account with an email and password. They can then sign into Final Draft 13, using up one of the available seats in your license.
You can also remove access to a user via this dashboard, as well as organize your users in various ways.