Sorry, I got confused by the impression that you had troubles with converting a normal album into a shared album and then to add persons to share with etc. etc.
But apart from temporary troubles at your Google server, I never have had any problems in my Chrome-browser with adding any pictures to any albums that I have created myself (not shared by others).
And I often do that immediately after a new upload has ended, because then it is asked if you want to add to any album. But later you also can work the other way around: first open the album and then press the tiny + to add from computer or library.
Maybe you should try another browser because there often are strange problems with Firefox.
Or do you work with the Backup & Sync app on your computer?
Or are you trying this in the app on your phone?