There currently is not a way to bulk load user IDs into a custom group in SAS Viya. We use Active Directory (LDAP) groups to get access to the SAS Viya system, but use custom groups to control access to specific reports.
However, other than selecting them one at a time in the SAS Environment Manager GUI or doing one line at a time using the sas-admin add-member CLI tool is not the most efficient way to do this. This is especially true in our environment where we could have 1000+ users that need to be added to one of the custom groups. Having the functionality to be able to import a list of user IDs would be a HUGE help in this process!