I wondered if you would still encounter the issue if you invited several users at once. When going through the invite process you should be able to enter several email addresses consecutively in the email address field separated by semicolons. Could you try that and see if it helps?
Having this exact problem now. I am trying to invite 29 people to collaborate. I've successfully (and painfully) been able to add 18 people. Now I am at a complete standstill. Stuck. What's weird is that every few tries, I get this "game" verification where I have to solve visual puzzles to prove I'm human. But after jumping through those hoops, nothing seems to happen. Trying to add multiple people at once just makes it freeze altogether. The only thing that has worked at all is to go all the way out of libraries and refresh OR restart the computer. I have 7 more libraries to go after this first one, so I need a solution... soon.