add below lines, where user123 is uid that need su - privillage
user123 ALL=(ALL) /usr/bin/su -
user124 ALL=(ALL) /usr/bin/su -
user125 ALL=(ALL) /usr/bin/su -
user126 ALL=(ALL) /usr/bin/su -
user123% sudo su -
We trust you have received the usual lecture from the local System
Administrator. It usually boils down to these two things:
#1) Respect the privacy of others.
#2) Think before you type.
Authorized access only!
Friday, September 5, 2008