The only requirement is to have the user’s email address.
Click on the top corner (profile icon), then click the “Invite a Team Member” link:
To invite a new user to the Company, simply enter their email address in the “Email address” box and click the “Send Invitation” button. This will send an email to the user, with a link to activate their account and join the Company’s team.
The invited user will show in a new panel titled “Mailed Invitations”. To re-send an invitation, click the X button and re-submit the email address in the “Send Invitation” panel.