Bash si el usuario existe en un grupo, luego agregue
#!/bin/bash
# init
USERID="$1"
#....
/bin/egrep -i "^${USERID}:" /etc/passwd
if [ $? -eq 0 ]; then
echo "User $USERID exists in /etc/passwd"
else
echo "User $USERID does not exists in /etc/passwd"
fi
# ....
Pleasant Porpoise