Because I noticed something, I will add it.
I had been thinking of only two examples Erik expressed, so I answered like that. But I could think more deeply by thinking of other examples.
うちのパーティーに来<き>ませんか。(招待<しょうたい>invitation)○(correct)
Why don't you come to the party at my house?
うちのパーティーに来ましょうか。 ×(incorrect)
お宅<たく>のパーティーに行きませんか。 ×(incorrect)
お宅のパーティーに行きましょうか。(申し出<もうしで>proposal)○(correct)
Shall I come to the party at your house?
I can say the following by this.
※The agent/actor in the sentence finished by ませんか is the hearer and the agent/actor in the sentence finished by ましょうか is the speaker.
Now I consider the sentences in question anew here.
I wrote "あしたいっしょにえいがをみませんか。Why don't we watch a movie together tomorrow?" and "あしたいっしょにえいがをみましょうか。Shall we watch a movie together tomorrow?" I think that these translations are natural in English. But I think that there is basically a difference like that in these sentences grammatically. When the agent is わたしたち(we), the difference does not appear clearly. But I think that there is still the difference between them. And It is related with my writing " We use みませんか/みないか when we don't know the answer will be "Yes" or "No". We use みましょうか/みようか when we expect the answer will be "Yes".
By the way, Hilary Metcalf asked me " would you say that the first is like an invitation, but the second is just a suggestion?". And I answered " No. Both of them are 誘<さそ>い・勧誘<かんゆう> (solicitation) and 提案<ていあん>(suggestion・proposal)." because the place we watch a movie is usually a movie theater. If the sentence is "あしたわたしのうちのえいがかんしょうかいにきませんか<Why don't you come to a movie appreciation meeting at my house tomorrow?>", you can say "invitation", but invitation is a kind of solicitation or suggestion.