ClientContext clientContext = new ClientContext("http://MyServer/sites/MySiteCollection "); GroupCollection collGroup = clientContext.Web.SiteGroups; Group oGroup = collGroup.GetById(6); UserCreationInformation userCreationInfo = new UserCreationInformation(); userCreationInfo.Email = "alias@somewhere.com"; userCreationInfo.LoginName = @"DOMAIN\alias"; userCreationInfo.Title = "John"; User oUser = oGroup.Users.Add(userCreationInfo); clientContext.ExecuteQuery();