It's easy. Just enter the attorney's unique PacerPro email as a secondary recipient of email notices. You can do this by accessing the attorney's profile on the websites of the courts where she is admitted.
The courts' websites are listed at https://www.pacer.gov/psco/cgi-bin/links.pl
Note that you cannot add secondary email addresses to district courts via the PACER NextGen site - you have to enter it on the individual courts' CM/ECF sites.
An attorney can find his or her PacerPro email address at https://app.pacerpro.com/accounts/edit.
PacerPro account administrators can see all their firm attorneys' PacerPro email addresses at https://app.pacerpro.com/organization -> users by clicking "Edit" next to the attorney's name.
NOTE: you cannot add secondary recipients to attorney profiles in EDNY through the court website.
To add an address to EDNY, you must contact the court directly at email@example.com, specifying the attorneys name and bar code. A bar code for the purposes of EDNY is the attorney's initials (first and last) and the last 4 digits of their Social Security number. For example, John Smith, SS# 123-45-6789 would be JS6789.