After purchasing a Resilio Sync Business license, it's possible to add more seats there if necessity arises. License Owner can do it from Sync app: open License settings and click on "Add more seats", or go directly to License Management Console
In License Management Console select "Add more seats" option. Only scale of +5 seats is available. Reducing seats is not supported.
After updating the number of seats, restart Sync. It shall be updated in the application automatically. If that did not happen, download the updated license key file and apply it on the current License Owner.