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You can save unlimited photos on Google Photos if you set your settings to upload to "High quality" as opposed to "Original quality." If you're using a Google Pixel phone, you may upload unlimited photos at original quality.
You can have multiple people manage your account by setting up a brand account and adding people as members to that account. (You can do the same with YouTube.) However (unlike YouTube), the iOS and Android apps currently do not support brand accounts, so you will not be able to manage your photos through these apps.
Anyone can share an album with anyone else. You can also set your albums to allow anyone to upload to an album. However, only the album owner will be able to edit the album.
When you share an album, you can select, "Create link," which will give you a link you can share with anyone. However, that link redirects. Be sure to share the link that Google Photos gives to you, not the one that appears in your browser bar.
Google Photos will show who uploaded the photo, but it won't show who actually took the photo. For the simplest use case (create an album, have lots of people share to it), this is fine. But for a more complicated (but common) use case, this causes problems. For example, I often create an initial internal photo album, where people upload photos, then create a second curated public album, where I select the best of the uploaded photos. In order to share photos that other people uploaded, I need to import them into my library, then add them to the new album. Unfortunately, in the new album, the photos looks like they came from me, not the original uploader. This is technically true, because I was forced to re-upload the photos, but it's not the desired behavior.