Only Admins and Owners can invite a user.
Click your email address in the top right of your home dashboard
Click the name of your organization under Manage Teams
Select the People tab
Click on Invite Your Team
Remember to select the appropriate permission level, and which Groups they should have access to.
The user has 24 hours to accept your invitation. If they do not accept the invitation within 24 hours, you will need to send them a new invite.