NOTE: you must have created an organisation before you add members. Please see the article How do I Create a new Organisation? to create an organisation.
From Memberships select the Members tab.
Click Add New Member.
Enter all the required information. To add the member to the organisation you created, click on the magnifying glass next to Organisation.
Click on the link before the organisation you created to select it.
Click Save. The member will be added to your organisation.