public async Task <IActionResult> OnGet(int invitationId, string email, string returnUrl = null)
        {
            // Return if invitation is already accepted
            Invitation invitation = InvitationsRepository.SearchFor(x => x.Id == invitationId).Include("User").Include("Event").FirstOrDefault();

            if (invitation.InvitationAccepted)
            {
                // Display page?
                return(RedirectToAction("Index", "Home"));
            }

            ApplicationUser invitee = await _userManager.FindByEmailAsync(email);

            if (invitee == null)
            {
                // If user doesn't exist, redirect to Register page
                return(RedirectToPage("/Account/Register", new { area = "Identity", returnUrl }));
            }

            DinnerEvent        dinnerEvent          = DinnerEventsRepository.GetById(invitation.Event.Id);
            EventParticipation inviteeParticipation = EventParticipationsRepository.SearchFor(x => x.User.Id == invitee.Id && x.Event.Id == invitation.Event.Id).FirstOrDefault();

            if (inviteeParticipation == null)
            {
                inviteeParticipation = new EventParticipation {
                    User = invitee, Event = dinnerEvent
                };
                EventParticipationsRepository.Insert(inviteeParticipation);
                EventParticipationsRepository.SaveChanges();
            }

            invitation.InvitationAccepted = true;
            InvitationsRepository.Update(invitation);
            // Check if user has more pending invitations and make them invalid
            var otherInvitations = InvitationsRepository.SearchFor(x => x.User.Id == invitee.Id);

            foreach (var otherInvitation in otherInvitations)
            {
                otherInvitation.InvitationAccepted = true;
                InvitationsRepository.Update(otherInvitation);
            }

            InvitationsRepository.SaveChanges();
            // Add contact to Mailjet Contacts, save ContactId
            string apiKey    = Configuration?.GetEmailSettings("apiKey");
            string apiSecret = Configuration?.GetEmailSettings("apiSecret");
            // Save list recipient id to database
            long listRecipientId = await EmailSender.AddContactToContactListAsync(apiKey, apiSecret, invitee.ContactId.ToString(CultureInfo.InvariantCulture), dinnerEvent.ContactList.ToString(CultureInfo.InvariantCulture));

            invitee.ListRecipientId = listRecipientId;
            await _userManager.UpdateAsync(invitee);

            EventParticipation inviterParticipation = EventParticipationsRepository.SearchFor(x => x.User.Id == invitation.User.Id && x.Event.Id == invitation.Event.Id).FirstOrDefault();
            // Create a new team
            Team newTeam = new Team
            {
                Partner1         = invitation.User,
                Partner2         = invitee,
                SelectedCourse   = inviterParticipation?.SelectedCourse,
                AddressStreet    = inviterParticipation.Address,
                AddressNumber    = inviterParticipation.AddressNumber,
                FullAddress      = inviterParticipation.Address + " " + inviterParticipation.AddressNumber,
                AddressAdditions = inviterParticipation.AddressAdditions,
                Phone            = inviterParticipation.Phone,
                DinnerSaver      = inviterParticipation.DinnerSaver,
                Event            = invitation.Event,
                Allergies        = inviterParticipation.Allergies,
                City             = dinnerEvent.City
            };

            TeamsRepository.Insert(newTeam);
            TeamsRepository.SaveChanges();
            // Send the email
            await EmailSender.SendMailjetAsync(apiKey, apiSecret, 1197519, "Deine Einladung wurde angenommen", "*****@*****.**", "Das Großstadt Dinner Team", invitation.User.FirstName, invitation.User.Email, invitation.User.FirstName + " " + invitation.User.LastName, "", invitee.FirstName, invitation.Event.EventName, invitation.Event.EventDate.ToShortDateString());

            TempData["StatusMessage"] = @"Du hast die Einladung erfolgreich angenommen. Die Teamdaten deines Partners wurden übernommen. Ihr seid nun vollständig angemeldet.";
            return(RedirectToPage("./RegistrationData"));
        }