public GroupEditPetitionCoauthorsViewModel(Petition petition)
        {
            if (petition != null)
            {
                if (petition.GroupId.HasValue)
                    GroupId = petition.GroupId.Value;

                EditPetitionCoauthors = new _EditPetitionCoauthorsViewModel(petition);
            }
        }
        public GroupEditPetitionCoauthorsViewModel(Petition petition)
        {
            if (petition != null)
            {
                if (petition.GroupId.HasValue)
                {
                    GroupId = petition.GroupId.Value;
                }

                EditPetitionCoauthors = new _EditPetitionCoauthorsViewModel(petition);
            }
        }
        public ActionResult InvitePetitionCoauthor(_EditPetitionCoauthorsViewModel model)
        {
            if (!Request.IsAuthenticated)
                throw new AuthenticationException();

            PetitionCoauthorContainer data = new PetitionCoauthorContainer
            {
                PetitionId = model.PetitionId,
                UserName = model.UserNameForInvite
            };

            VotingService.InvitePetitionCoauthor(data, UserContext.Current.Id);

            return Redirect(Request.UrlReferrer.PathAndQuery);
        }