public async Task <IActionResult> GetUsersWithoutGroup(int id)
{
var usersFromRepo = await _repo.GetAllUsers();
var groupUsersFromRepo = await _repo.GetGroupUsers(id);
var usersWithGroup = groupUsersFromRepo.SelectMany(u => usersFromRepo.Where(x => x.Id == u.UserId)).ToList();
var usersWithoutGroup = usersFromRepo.Except(usersWithGroup);
var usersToReturn = usersWithoutGroup.ToList();
return(Ok(usersToReturn));
}