The Batch Script will allow you to bulk add users to a specific group.
You will need to add the user IDs into the batch script, I've left examples called "USERID1 USERID2", you will need to leave the quotation marks in your batch script so be careful not to remove them.
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BAT-AddUsersToGroup[NameOfGroup].bat
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@ECHO OFF
CLS
ECHO -----------------------------------------------
ECHO Adding user(s) to the group NameOfGroup@TLD.com
ECHO -----------------------------------------------
cd C:\Gam\
gam update group NameOfGroup@TLD.com add users "USERID1 USERID2"
ECHO --------------------------------------------------------
ECHO Finished adding user(s) to the group NameOfGroup@TLD.com
ECHO --------------------------------------------------------
Pause