$ cat test.txt 
admins = anand,satheesha,user1,user2,user3,user4,user5,user10,vishal
nonadmins = read-only
@admins = rw
@nonadmins = r

#!/bin/bash

check(){
    if [ -z "$1" ];then echo "No user input exiting..";exit;fi
       }

printf "\nOption Availble :\n\t\t  1. Add User\n\t\t  2. Delete User\n\
Usage :\n\t   Add User----> 1 username filename\n\tDelete User----> 2 username filename\n\n\n\tAnswer  : \t"

read inp usr fil
check $inp;check $usr;check $fil

ex=$(grep "$usr" $fil)
if [ $inp -eq "1" ] || [[ $inp -eq "2"  &&  ! -z "$ex" ]];then 

    awk -F, -v arg=$inp -v usr=$usr '

    function remove(us,line){
                                u1=","us; u2=us","
                                error="User Not Found"
                                return ( ((gsub(u1,x,line)) || (gsub(u2,x,line)))==1 )?line:error    
                            }
                            {         
                                print (arg==1)?(/^admins/)?$0","usr:$0:\
                                      (arg==2)?(/^admins/)?remove(usr,$0):$0:"Error"
                            }' $fil

else 
    echo "It seems username not exist in file"    
    exit;
fi

$ bash user.sh

Option Availble :
          1. Add User
          2. Delete User
Usage :
       Add User----> 1 username filename
    Delete User----> 2 username filename


    Answer  :     1 dummy_user123 test.txt
admins = anand,satheesha,user1,user2,user3,user4,user5,user10,vishal,dummy_user123
nonadmins = read-only
@admins = rw
@nonadmins = r

$ bash user.sh

Option Availble :
          1. Add User
          2. Delete User
Usage :
       Add User----> 1 username filename
    Delete User----> 2 username filename


    Answer  :     2 user3 test.txt
admins = anand,satheesha,user1,user2,user4,user5,user10,vishal
nonadmins = read-only
@admins = rw
@nonadmins = r

$ bash user.sh

Option Availble :
          1. Add User
          2. Delete User
Usage :
       Add User----> 1 username filename
    Delete User----> 2 username filename


    Answer  :     2 akshay test.txt
It seems username not exist in file