Navigate to 'Settings' --> 'User Management' and press 'Invite User'.

Fill in an email address and select a team role. Optional you can add the user to a group. Press 'Send Invite'. An email will be send to the users email address with further instructions. 

In the tab 'Pending Invites' you can see the invite(s) that are send to the users and have not login to Relayter yet.