California State University, Sacramento
Commonly a student gets an "authentication error" even with the correct username and password. In almost every case this is becuase the student's username is being entered with a capital letter (typically the first letter).
The SacLink ID that is plugged in MUST be all lowercase for WeBWorK to work. The common culprits are:
In all of these cases, have your student log off of the Sac State system. Provide them with a link to your WeBWorK course, and ask that they login paying attention to not capitalizing their username.
If you continue to have problems, you might want to ask for a screenshot of their username being plugged in (to verify it is not capitalized).
The process for bulk adding students is explained here.