To Invite new users follow these steps;
Step 1: Go to: the Settings Icon > Organization Settings > Users & Groups
Step 2: Click on the Green "Invite User" button
Step 3: Enter the new users email address
Step 4: Select the User Role for the new user. (Quality / Normal / Basic) For more information on User Roles click Here
Step 5: Click the "Send Invitation" button
Keep in mind that the default expiration time for an invitation is 14 days.
To re-send invitations, follow these steps;
If you need to re-send an invitation,
Step 1: find the user in question under Users & Groups.
Step 2: Click on the user's name and you will be brought to their profile.
Step 3: Click "Resend"