1. Go to the organization's page. Click on < Members >.
2. Click on < Add members >.
3. Select the role for new members (you can always change it later). 
4. Enter your collaborator's email address in the gray box.
5. Click on < Add to List >.
6. Click < Add > to complete this step.

Check out this article to find out what each role is in your organization.
To find out where to find your organization's membership page go here.