Click your account name in the top right hand corner.

Click "Your Settings”.

In the tabs on the left had side click the tab “User Managment”.

Under “Send Invitation” type the name and email address of the user you wish to add. 

After providing their name and email you must click the box stating that you understand that each addition user that accepts an invite will be an additional 5$ a month.

All of the users who have accepted an invite will be listed under “Business Members”.