how to su - from non root acount to non root account

HI i am trying to give su access to some users say X Y and Z to a account AB . I am able to give them su access to root with the help of sudoers file but i want to give them password less access to AB account which i am not able to do .
I want to this

when user X fires "su - AB" he is not prompted for password.

please help.

What, exactly, did you put in the sudoers file?

UserX ALL=(ALL) NOPASSWD: ALL

Please , can someone help me in this .

So please post the command you run and the error message you get, if any.
I'm expecting:

sudo su - AB

Also, what says

sudo -ll

?

when user X fires this cmd sudo su - AB

it asks for a password ...

UserX ALL=(ALL) NOPASSWD: ALL

This will help in giving the User X root acess i want him to to login to accoutn AB without password.

i tried this
User_Alias ABC = X,Y

ABC server = NOPASSWD

but it did not work ..
can any one help me on this

rishiraaz,

sudo file entry looks fine...but can you post us the output of sudo -ll after logging via sudo su - AB from user x/y

Which will help us in identifying the exact issue.