A wedding album outside a shared library
A wedding Chest is not a Google Photos library or an iCloud shared album. Guests join with a name.
Google Photos and iCloud are libraries tied to those accounts. OurChest does not import a Google Photos library or an iCloud library in the first release. People add photos from the phone into the Chest you invited them to.
There is no password, and guests do not pick a cloud provider on the way in. They join with a display name. The photos in the Chest are the files those phones uploaded.
Copies to Google Drive, Dropbox, and OneDrive are a later release. They are not part of the first release, and this page does not say you can turn them on today.