Mongoose: Mercenary has a chapter devoted to Tickets. It begins by saying:
Tickets are legally sanctioned, but not always publicly known, contracts between one party (the employer) and the mercenary unit involved (the employee). Each ticket is backed by the local authority that must be present when it is drawn...
...Although just having a ticket does not necessarily mean that the authorities (the legal agency in the vicinity of the ticket's execution) will accept it, merely that someone with the right lawyers, money and power will stand behind the mercenaries if need be. Having a properly drawn up ticket with a group of mercenaries will not make them immune to legal retribution, but it will protect them most of the time."
The chapter then goes on to explain how a ticket is drawn up and what DMs affect its likelihood of success, escape clauses, repatriation bonds, etc.
I'm not aware of such detail elsewhere in canon.
So, it looks like you only need a licence to fight (a ticket), and don't need one to recruit, train, and advertise. Until you fight, you're just an ex-military old boys club.