I have a business license. It's a requirement for my county. It's for my company and covers all the "on-the-side" activities that company conducts.
When operating at airports, I'm sensitive to the rules and policies of those airports. In my area the airports place restrictions on independent operators to solicit/advertise and use the airport facilities for conducting their business, unless those operators jump through specific hoops that the airport calls out.
They do not place any restrictions on a tenant of that airport hiring any CFI they want as long as the "work" takes place in that owner's airplane.
Examples of "ok" behavior:
Coming into the airport, meeting a client, and going out to his airplane and giving him instruction in it.
Helping a client work through a weather briefing in the flight planning room.
Examples of "not ok" behavior:
Putting up signs/notes advertising instruction.
Teaching a ground lesson in the airport lobby or lounge, or out on the picnic table.
One airport manager called me a "rogue" flight instructor once, but he said it with a smile. Honestly, I understand that the airport has tenants who pay the airport for the ability to conduct their business there, and I've found that with a good attitude of mutual respect, they don't feel threatened by the work I do, and I don't feel "slighted" by the fact that I can't directly/openly compete with them without putting the same skin in the game that they have.