Adding photos to an album in Photos?

Which system version do you have installed, Nathan?

On macOS 10.13.3, when I drag a photo to an album in the sidebar, the cursor will change to a ➕, and the photo will be added to the album. It will depend on your Preferences for the sort order of the album, where the photo will be added. It will not be added necessarily at the end of the album.

One more thought.

I just remembered, that "Add to album" or dragging to an album occasionally does not work for me either since the 10.13.3 update.

In that case I restart my my Mac and all will be back to normal.

but if anyone has recommended alternative photo library managers, I'm really interested in what people are using to replace Aperture and iPhoto.

I do not want to use an alternative Photo Manager, because I want to iCloud support, but I am using a different Browser for m Photos Library. Power Photos is better at showing metadata. It has a list view like aperture, showing the metadata nicely in columns, and can show two lines of metadata below the thumbnails. The background can be turned black. But it is only a convenient Browser, it cannot be used as an editor. When I want to edit a photo, I crtl-click it in PowerPhotos and use the command "Select in Photos". Then I can work on the photo in Photos.

I works for many of us. Do note that you can only drag a photo to an album you have created yourself under My Albums or a Shared Album, not one of the predefined system albums.