The signup user will be assigned as the owner of your organization. Each account will only have 1 owner. Only the owner of an organization will have access to account and billing settings.
There are 3 different kinds of roles for an account user.
Owner: Have complete access to the account. The user used for sign up will always be assigned with Owner role. Each account will only have 1 owner.
Admin: Have access to most configurations within an account. Doesn't have access to account and billing settings.
Read Only: Have read-only access. A recommended role for non-tech savvy users.