When you create a new organization, you are the only one with an account in it. You can invite others to join your organization. All you need is their email address. When they receive the invitation, they will make the choice to accept it, reject it, or ignore it. If they accept the invite, they will be able to create an account in your organization. If they reject it, then the invitation is no longer valid, and they will need a new invite if they change their mind later. An invitation is only valid for 7 days before it expires. Once expired, a new invitation will be needed to join.
Navigate to Manage Organization by clicking on the org icon in the lower left-hand corner of the page, above your avatar, and select Manage Users from the navigation panel.
Click the plus-sign icon in the upper-right to send invitations. Use the ellipsis menu at the end of each row to take actions on an individual account. User accounts can be filtered by name, email address, and account status.