public Task <Result <Unit> > Get(RegisterUserRequest key)
 => _createUser.Get(key)
 .ThenConcatWith(userId => _createGroup.Get(new CreateGroupRequest {
     GroupName = $"{key.Email} Personal"
 }))
 .Then(ctx => _addUserToGroup.Get(new AddUserToGroupRequest {
     UserId = ctx.Item1, GroupId = ctx.Item2
 }))
 .Then(_ => new Unit());