|I agree with Razor2.3,|
I.T. responsibilities can be very fragmented depending on company size, data sensitivity or even due to micromanagement.
To be given the ability to create / maintain user groups in my opinion is a high level I.T. responsibility.
On one hand I would feel privileged being given the responsibility, on the other, I would ask if am I paid enough for the accountability.
Job descriptions don't mean much these days as there is usually a clause that indicates that as a condition of employment that duties include tasks nominated by the employers representative be followed within reason.
... If you are concerned broach it with your employer.
... If you can't be bothered putting in the extra mile there are probably many willing and able.