Because you access an organization by logging in to a user account, each of your team members needs to create their own user account. After you have usernames for each person you'd like to add to your organization, you can add the users to teams.
If you need greater control over the user accounts of your organization members, consider Enterprise Managed Users. Enterprise Managed Users is a feature of GitHub Enterprise Cloud that provides even greater control over enterprise members and resources. With Enterprise Managed Users, all members are provisioned and managed through your identity provider (IdP) instead of users creating their own accounts on GitHub. Team membership can be managed using groups on your IdP. Managed users are restricted to their enterprise and are unable to push code, collaborate, or interact with users, repositories, and organizations outside of their enterprise. For more information, see "About Enterprise Managed Users" in the GitHub Enterprise Cloud documentation.