General :: Adding Users To Existing Group

I've been asked by my professor to add the list of users to a linux server (not sure of the OS type I think he said debian) but anyway. He gave me this script to add users.

Code: #!/bin/bash
# Script to add a user to Linux system
if [ $(id -u) -eq 0 ]; then
read -p "Enter username : " username
read -s -p "Enter password : " password
egrep "^$username" /etc/passwd >/dev/null
if [ $? -eq 0 ]; then
echo "$username exists!"
exit 1
else
pass=$(perl -e 'print crypt($ARGV[0], "password")' $password)
useradd -m -p $pass $username
[ $? -eq 0 ] && echo "User has been added to system!" || echo "Failed to add a user!"
fi
else
echo "Only root may add a user to the system"
exit 2
fi

I need to see if I can get this script to read a file that list the usernames and their passwords using the pipe command (or some similar command) so I can just do it in one batch. I've done some searching but there are so many vairiations of the code that I've confused myself. Also, I'm not too familiar with linux, it's been a few years since I've used it but in the prior script, I need to add the users to an existing group named "forensics". Which line would I change/add in order to do this?