Shell Programming and Scripting

I have a file below which has a list of users and roles with each row having unique combination of users and roles.

USER1 ROLE1
USER1 ROLE2
USER2
USER3 ROLE1
USER3 ROLE2
USER3 ROLE3
USER4 ROLE2
....
....


I am trying to create a script which sorts the above file to have all the roles in one line delimited by commas per user to look like below:

USER1 ROLE1,ROLE2
USER2
USER3 ROLE1,ROLE2,ROLE3
USER4 ROLE2
....
....


I was able to do this with the script below but when the input is very large it takes very long and takes up cpu usage so I was wondering if someone could tell me a faster way to achive this.

Code:

#!/bin/ksh

INPUT=$1

cut -d' ' -f1 $INPUT | sort -u > USER_LIST

while read USER
do
**** ROLES=$(echo "`grep $USER $INPUT | cut -d' ' -f2 | tr '\12' ','`" | sed -e
's/,$//g')

**** [[ $USER != $ROLES ]] && echo $USER $ROLES
**** [[ $USER = $ROLES ]] && echo $USER

done < USER_LIST