Do guests need an app or an account to add photographs?
No. They scan the code, the page opens in the browser on their phone, they choose pictures and upload. There is no installation step, which matters when your guests range from eleven to eighty-one.
Can strangers see the photographs?
The page lives at a link you send, not in a search result, and you can set a password so only guests you give it to can open it. The printed code alone does not hand the gallery to anybody who wanders in off the internet.
Can I add photographs myself from my own phone?
Yes. You are a guest at your own party: open the page and upload from it the same way anyone else would, including the pictures of the room that nobody else photographed.
Will the photographs be on the page before I publish it?
Nothing guests can see exists until you publish, and that applies to the gallery as much as the invitation. Until then it is a private draft, so build the page, test the code yourself, and publish when you are happy with it.
What about the cousins who will not scan anything?
Some photographs will never make it onto the page, and that is honest to expect. Ask one person per family group to be the uploader for the rest of them, and keep the code somewhere visible rather than in your bag.