Hi S R,
The build in implementation does not expose this functionality. First you need to find a way to set from which newsletter group this user will be unsubscribed - say A. Then you have to keep this user in group B. The only way to do this getting the user again and adding it to B.
ISubscriber subscriber = dataManager.GetSubscriber ( subscriberEmail );
if ( subscriber != null )
Guid groupID = new Guid("89B46498-9160-4199-B584-B434387F37ED");
IGroupSubscriber GSubscriber = dataManager.CreateGroupSubscriberRelation(groupID , subscriber.SubscriberId);
the Telerik team