The short answer is yes, it’s not a problem.
The long answer is “it depends”.
The rules vary depending on the country you’re visiting, whether it’s day trips or you’re actually staying there, how long you’re staying for and how often you’re doing it.
On top of that, it also depends on what qualifications you hold and how well-recognised they are, plus what your insurance will allow.
Going over the border for occasional day trips is a non-issue. Staying for a couple of nights, bringing your own clients with you is fine, but in theory you’re supposed to notify the host nation in advance. Staying longer than 2 weeks (guide, not clients) or doing frequent stays requires more detailed pre-notification.
I’m insured through the French bike-guides scheme at the moment, which is “anyone, anywhere” insurance (i.e. I can guide clients from any country, in any country). That’s not actually totally straight-forward to obtain – e.g. I’ve never really used the insurance offered by the UK association as it only allowed me to guide clients who were UK residents (which would be a major problem as we have a lot of clients from e.g. Russia and South Africa as well as it being a bit ridiculous to be based in France and not able to work with French clients).
