What is the difference between the position IT manager and group IT manager in terms of job description?

I.T.
Manager is generally mangignt I.T.
for an organiation or Part of Big Organization.
Group I.T.
Manager post are in Big wide Spread organization with more diversified and complex technical wise Spread.
Group I.T.
manager have more bird eye view on I.T.
and have more managerail responsibility..

I would say that both of them will have the same job description with different scopes. The It manager will align his/her work the goals of the group's IT manager whicle the group IT manager align his/her work to the group's goals.

A single business concern hires IT manager titled person as this person is responsible for this single business entity only.
A group which has several business concerns or entities will need Group IT manager to handle IT matters for all the separate business entities working under the group umbrella.