This feature is only available for the Business plan.
If you are not part of any other organization you can add members to the organization as follows: 

1. Click on "Admin management". 

2. Open the users page

3. Click on "Invite". 

4. Enter the user details and choose their role. 

5. The users must accept the invitation for them to be added to the organization But please note that you cannot add a user that is member of another organization.