I was thinking of two ways, however one applies, but I'll explain.
You can use a loop and open a reference file which has line by line user emails and the loop iteratively calls /opt/csw/mpack with the last argument substituted as you read the reference file.
The one that applies would be that you have a file which has one line, space delimited user emails, no loop and you merely use the entire contents of that file in a single substitution for one call to /opt/csw/mpack. The reason would be so that all recipients would see the full list of other recipients, as you've said here.
A caution is that if that list of recipients becomes really large, it could cause the command to be too long for your shell.