I had the same issue and got help from support. Here is the workaround – have the person you invite open a browser in incognito mode, this way there is no interference from other google accounts (especially if you are joining on behalf of others). Go to fi.google.com/signup. Login with the account invited. Then you will see the options to start your own plan or join the one you were invited to. Choose the to join the plan you were invited to This way works! There was no insight on what the issue is, but at least there is a workaround. Good luck!