Repentance From Backbiting and Informing Victim
Question

                I have committed backbiting involving a person or a group of people, or have unjustly slandered some people regarding things with which they didn't have anything to do. Is it a condition of repentance that I should inform them and seek their pardon? And if this is not required then how do I repent?
Praise be to Allah.The answer will depend on 
    the nature of the interests of the people involved. If the case is such that 
    if he were to inform them of what he had once said about them, it would not 
    be likely to increase their anger, then, he should do that and seek their 
    forgiveness. He may then tell them - in general terms, and without offering 
    details - that he had wronged them in the past by expressing such sentiments 
    about them that were not right and just, and that now he has repented and 
    seeks their forgiveness. 
But if the case is the 
    opposite, that is, the fear is that if he told them of the previous 
    backbiting or slander, they will - and in most cases that is most likely to 
    happen - be angry with him, or will not be satisfied with the general terms, 
    rather, will demand to know all the details, which is sure to provoke them 
    to anger, then, in such a case, it is not obligatory on him to let them know 
    and seek their forgiveness for the sharee'ah (Islamic law) does not order 
    adding insult to injury. And informing a person who was in a happy mood, of 
    things that will evoke feelings of distress or anger, is something against 
    the purpose of the sharee'ah, which enjoins the spread of happiness and 
    brotherliness. In fact, sometimes such action may give place to animosity 
    where there was love. In such situations it is enough to repent in the 
    following manner. 

    Evoke 
    in oneself the feeling of regret, and seek forgiveness from Allah 
    accompanied by the conviction that it was an abominable and unlawful thing 
    in which he had indulged. Try and make up for what he had said earlier 
    before others by denying the presence of things and qualities he had alleged 
    were in them and clear the person or persons involved. Follow up by 
    mentioning and asserting good qualities of those he had spoken ill of in the 
    presence of the same people in whose company he had slandered them. Defend 
    the people he had slandered when they are slandered in his presence. Pray to 
    Allah in private to forgive the sins and shortcomings of those he had 
    wronged. (See Madarij Al-Salikin, and Al-Mughni with explanatory notes for 
    further elucidation). 
You must note, dear 
    brother Muslim or sister Muslimah, that there is a difference between the 
    material rights of the people and non-material rights, between backbiting 
    and defamation. When the material rights are returned to the people, they 
    are put to some use and cause increase in happiness, and therefore they must 
    not be put under the mattress. Whereas non-material rights, such as those 
    concerning a man's honor, when returned to the people can only cause 
    increase in distress.

    And Allah knows best.