Add photos/videos to albums
Picasa used to upload folders from a computer to albums with the same name, even in batch, and (optional) sync computer folders with Google Photos. The actual Photos team however did not find it necessary to implement such option. Adding photos to albums in Google Photos is now always a manual action.
Add photos/videos from Google Photos
For photos that are already uploaded to Google Photos, you can:
Select (multiple) photos anywhere in Google Photos, and add them to an album via the + icon (Add to)
Tip: on a computer you can find an existing album by using the Find function (Ctrl+F) of the browser.Or start from the album and use the "Add photos" icon (icon with a small +)
This brings you to the timeline where photos that are already in the album are selected.
Warning: deselecting will remove these photos from the album!
Tip: you can also "Select from computer" or use the search box to find photos.
Adding from a folder on a computer to an album
When you upload photos manually from a computer to Google Photos, you get a pop-up asking whether you want to add the uploaded photos to an album. When you missed that option, you still find newly added photos on top in Recently added.
Warning: Photos/videos that were already uploaded will NOT be uploaded again, and NOT added to the album. Therefore I recommend the method described hereafter.The better method is to upload directly to the album, either by drag & drop into the open album or by using the "Add photos" icon in the album. When using this method, ALL photos should be added to the album, even when you try to upload duplicates.
Tip: You can use this method also when the photos are already uploaded.
Adding from a mobile device to an album
When your photos are already uploaded, for example by using Backup, you can still:
Go to Library > Photos on device (Android only) or anywhere in the Google Photos app
Select photos you want to add to an album
Tap the + icon > Album or select an existing album
Warning: when you (accidentally) touch an album the photos are added immediately without confirmation. You will have to find them there, add to the correct album, and remove from or delete the wrong album.
You can also add photos to an album when they are not yet uploaded to Google Photos. That should also upload them to the Google Photos timeline.
Warning: When uploading manually (3-dots > Back up now) or adding not-yet-uploaded photos to an album, the settings under "Backup" on the device are not valid:For upload size it uses the setting in https://photos.google.com/settings
It may use mobile data, regardless the setting in Backup and sync.
Adding ALL photos to albums
Some users (like me) want ALL photos in albums. Former Picasaweb users already had ALL photos in albums when Google Photos was introduced in 2015. They could rather easily continue to add all newly uploaded photos to at least one album. It's more difficult when you start using Google Photos with a big collection.
Which photos are not yet in an album?
Adding a photo to one or more albums does not remove them from the Photos tab. How do you know whether a photo is already in an album?
The info of a photo in the web app https://photos.google.com shows all albums to which the photo was added. This info is missing in the Google Photos app on mobile devices.
Some recommend to hide the photos by moving them to "Archive". That is a not intended use of the archive (intended to hide "clutter"), and may have unwanted effects, like photos not being shared by partner sharing, not added to live albums or not used in Memories. I don't recommend that method, also because archiving from within an album is not implemented: you will have to select them again somewhere else. There are some "tricks" to find them though:
Search with the album title, but do NOT click on the suggested album. You may get a search result with the exact same photos when the title was specific enough, and can archive from here.
If that does not work, you could add something to the album title to make it unique, but then you will have to wait some time till the modified (or new) album title is indexed.
When you try to add photos to an album (starting from the album) it brings you to the timeline where all photos that are already in the album are selected. That allows to select more in the same environment, but also has the risk to remove photos from the album by deselecting them. I find this method too tricky.
How to be sure that ALL photos are in albums?
You can add photos to albums based on folders on computer or mobile device (see higher).
You can systematically add new photos to albums, and the backlog starting from the top (newest), note the date and continue from there next time.