Why am I unable to save a new Site User to my site?

When attempting to add a new user to your Site via SiteName > User Management > Site Users, you may be presented with the following message when trying to save the entry:
"Unable to save user. Please make sure you have filled out all required fields. Remember that the email address must be a proper email address and that the user name cannot be an email address. In addition, you may not use an email address or user name that is already in use in this site. This includes deleted users. If you are creating a new user, make sure to provide and confirm the password. "
While it may be because the required fields have not been completed or the email address is not a valid format, often this is presented because the email address is an existing email address for a user on that site.  You will first want to check to see if this email address is already in use.
When checking the list of active users, it may not immediately show the user's email address that you are trying to add - yet the error is still showing.  This may be because the user's email address may be linked to an inactive Site User - one you may have deleted at some point in the past.
To show the inactive users, go to SiteName > User Management > Site Users and at the top of the screen, click "Filter".  Then, click the checkbox to "Include Inactive" before finally, clicking "Filter".  This will show the list of all users, active and inactive and if the email address exists, it should be searchable.
This account can be reactivated, or if you plan to add this as a new user with the previous email address - change the email address on the old user before deactivating it once more.  Once done, you should be able to add your new user with that email address.
If you continue to receive this error or have any continuing troubles with this, please contact support.