In the sense of other people around the world, in different countries or cities, no, they cannot (at least by default). Smart DNS companies are protective of the account you create, and so the only way your service can be used by someone else is via theft. Of course, anyone whom you provide your account credentials to can potentially use your smart DNS. Still, it is important to highlight that all companies work with an account-based model – that is customers getting private environments to use products in. They are designed not to be used by anyone else but you.