Manually, you can add people or organizations by navigating to People > New Person. If you would like to add an organization, click the down arrow next to the New Person button and click New Organization.
However, your organization probably engaged with people before you started your nation. Feel free to follow our guide on how to prepare for your first import. If you get stuck or are having trouble, the NationBuilder Services team can also help import data and complete your first import for you for free.