controller.ex

defmodule MyApp.UserController do
  use MyApp.Web, :controller

  def create(conn, params) do
    attrs = %{email: params["email"]}

    case MyApp.Users.Invite.call(attrs) do
      {:ok, user} ->
        render_user(conn, user)
      {:error, :invalid_email} ->
        resp(conn, 400, %{message: "email is invalid"} |> Poison.encode!)
    end
  end

  defp render_user(conn, user) do
    {:ok, body} = %{
      id: user.id,
      email: user.email
    }
    |> Poison.encode

    conn
    |> resp(201, body)
  end
end

invite_user.ex

defmodule MyApp.Users.Invite do

  alias MyApp.User
  alias MyApp.Repo

  @create_params [:email]

  def call(%{email: email}) do
    with :ok <- validate_email(email) do
      email
      |> build_changeset
      |> create_user
    end
  end

  defp build_changeset(email) do
    params = %{email: email}

    %User{}
    |> Ecto.Changeset.cast(params, @create_params)
    |> Ecto.Changeset.unique_constraint(:email)
  end

  defp create_user(changeset) do
    changeset
    |> Repo.insert
  end

  defp validate_email(email) do
    case String.match?(email, ~r/@/) do
      true -> :ok
      false -> {:error, :invalid_email}
    end
  end
end