Your input file is in this format:
and you need to add these users to the system, is that right?
First thing to do in your script would be:
this sets the 'splitting' character to \n, so you can work with lines that have spaces. Then you could do something like this:
for x in `cat userlist.txt`
USER=`echo $x | cut -f 1 -d:`
PW=`echo $x | cut -f 2 -d:`
GRP=`echo $x | cut -f 3 -d:`
NAME=`echo $x | cut -f 4 -d:`
The lines xxxx are the ones I can't be sure of as I don't use Redhat, but check your man pages for scripts like adduser or useradd, addgroup or groupadd. To set the passwords, you might need to install expect or find a tool that sets passwords from the command line.