public void AddUserToGroup(string userId, int GroupId)
        {
            var group = _db.Groups.Find(GroupId);
            var user = _db.Users.Find(userId);

            var userGroup = new ApplicationUserGroup()
            {
                Group = group,
                GroupId = group.Id,
                User = user,
                UserId = user.Id
            };

            foreach (var role in group.Roles)
            {
                _userManager.AddToRole(userId, role.Role.Name);
            }
            user.Groups.Add(userGroup);
            _db.SaveChanges();
        }