If I upload photos, do guests see them instantly?

Only if the album is already published, and even then it's wise to think of publishing as the real gate. The recommended flow is to keep the album in draft while you upload everything, let Snaptide process the images and prepare the AI face discovery in the background, then publish when you're happy. That way guests never see a half-baked album with three photos in it. If you upload to an already-published album, the new photos do become accessible as part of that album — so publish when you're actually ready.