The Packard Companies - can't add employees into the site - server error

Hi, In Stage for the Packard Companies, every time we try to add in employees manually we are receiving an error message (please see screen shot attached). It looks like it is being saved in the table on the back end as I am seeing them in the SA utility, if I were to delete these employees, however, they are not appearing in the user interface.
Could you please let us know why we are getting this error message and help us to resolve this issue so that we can add employees into this site?
Thank you,
Robyn
Chitra Rao, Amannda Johnson, Ashley Marra