Hey Hedei. The short answer to your question is “of course” it would be fine to offer outdoor group classes! There is a market for just about any type of training and training venue. As some of the other posters have already mentioned, finding the clients, finding a location and making sure that you have proper permits etc. to make this all work, is your real issue/concern.
I love training clients outdoors, and I have to say, they love it too! Good luck!