Can't add all users

Hello,
I continue to have problems adding users. I added one user, who then had a problem activating her account, and so I removed her, thinking we could start fresh. I now can’t add her back - I receive an error message saying that this user cannot be added to my Omeka site.

Another user that I invited appears in the user list, but can’t access the site. Her user line is grayed-out. I presume it’s because she did not activate her account in a timely fashion when I sent the invite to her, but I do not know how to resolve this.

Thanks in advance for any assistance!