Planet Granite is open to the public just like most gyms. Just walk in with ID, fill out some paperwork, choose what type of membership you want, and then pay. The gym also offers classes that can be paid by the session without signing up for a membership.
You can sign a waiver online, I think there's a waiver tab when you click first visit. After that you have to go to their front desk with your photo ID and a form of payment.
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