Pantheist groups most often meet in natural surroundings. In inclement weather, they meet in each others' houses. If the group is too large for that, hiring would seem more sensible than devoting massive funds to a special building that remained empty most of the time. What pantheist circles do is up to each circle. What follows below consists only of outline suggestions.
Groups are percetly free to create their own formats and topics.
A shared exploration topic. Exploring, discovering, and getting to know and love better the natural world and the universe. Each week could be devoted to one theme, which might involve a field trip, bringing a natural object, exploring the elements, or pantheist ways of living. On indoor weeks one of the group would give a short talk on a different topic, not from a sophisticated scientific point of view, often perhaps just about the enthusiasm for natural form. The purpose is not just to understand the world, but to appreciate nature, ecosystems, natural form, as a way of reinforcing our feeling of unity and solidarity.