What is the extra visitors feature?
Basically, it's akin to a limited cell phone carrier plan; if one exceeds the limits of their plan, in lieu of shutting Justuno down, it will continue to run while incurring charges.
Login to your Justuno account, and activate the feature here:
Note: Charges do not roll over to the next usage cycle
NOTE: prices may vary based on discounts applied at purchase of your plan!