Just click on the "Invite" button
You get a pop up window to enter the email Id of the person you are inviting.
And then you need to choose if this person should be invited as a Member or a Guest. Here is how it will look just before you click on "Invite"
Inviting a Member
Or a Guest
An email will be sent out to them from where they need to "accept invite" or "join" the organization. Watch this video tutorial to know more.