-Members of Gendarmerie Internationale Inc. and Sovereign Security Solutions, both of which are private military contractor companies that threw their lot in with the Sons of Liberty after lengthy negotiations between the three groups. Both of these groups provide tactical and strategic expertise from combat around the globe during the third world war and the interim bush wars between feuding states and factions. They also have their own skilled combat and support personnel, most of which are military veterans. Their cutting-edge materiel, such as assault mechs and aircraft, have seen significant wear and tear during armed operations against the Empire, but they're still a potent arm of the Sons and are able to finance and supply the rest of the organization due to their status as multinational corporations

As for getting in with a closed borders policy, closing your borders closes legal entry. For anyone entering illegally what does it do? Nothing, its based entirely on the honour system in that case.
For a point, in both world wars, when borders were obviously closed and everyone was shitting themselves over the walls having ears and running around looking for spies, both the allies and the axis managed to place agents in the opposing countries. This is not star empires where one has to land on the planet and risk being shot down by space ships, getting a single person into a country, even in the modern days, isn't actually all that difficult...