You can invite other users to your organization via the + Invite User button, which is always visible in the top right hand corner of the application. Alternatively, you can use the Send Invitation button on the Organization Settings page.
Simply enter the email address of the user you would like to invite and click Send Invitation
The user will now receive an invitation email, which specifically mentions that you invited them, and be taken through the sign-up process
Once they accept the invitation, you will receive a confirmation email too