Channels are just names. In the Docs you will see that example:

channel "room:*", HelloWeb.RoomChannel

Then there is a join function:

defmodule HelloWeb.RoomChannel do
  use Phoenix.Channel

  def join("room:lobby", _message, socket) do
    {:ok, socket}
  end
  def join("room:" <> _private_room_id, _params, _socket) do
    {:error, %{reason: "unauthorized"}}
  end
end

(in this example the private rooms are always refused since the docs talks about implementing the authorization later on).

As long as your join function accepts _private_room_id as a room name (i.e. it returns ok), then your channel exists, basically.