This is most likely due to the user existing within the platform already. 

If you receive the error "An error occurred while inviting user@yourdomain.com. Please try again." while trying to invite a new user then it's most likely that the user already exists in the platform but may be associated to a different organization or account.  If the users email is not listed within SETUP>ADMIN>USERS then file a support ticket.

Another common reason a new user cannot be added is that SSO is configured for the tenant which prevents direct invitations from the platform to be available.  New users must be added via the SSO service provider.

For more information regarding SSO configuration, see:  Invite Users Via Single Sign On