Click on the Share button in the upper-right corner of the screen:
This will bring up the Sharing dialogue box:
Type in the email address of the user you'd like to invite, then click Show Options. Check the box to Send an Email Invitation. Select the permission group you'd like to add them to:
You can type in an optional message to the user in the large text box in the center. Once complete, click the Share button in the lower-right corner: