having two list box and need to compare the values

I need to compare the two sessions ,
in that one has actual list of users
for eg :
AAA
BBB
CCC

other session has list of added or removed users.
AAA
CCC
DDD
EEE
note: have removed user BBB and added DDD EEE
from the above list.. i want to send email to all users AAA CCC DDD EEE with the mail body DDD EEE are assigned as new users and BBB is removed from user list.
How can i achieve this by comparing the session. please help? thanks in advance..