I have recently upgraded my account to Pemium for Family. When I click the button to add or remove members, I only get to a black page. There is no possibility to send an invite to someone.
Apologies for the late reply, but maybe I can still help 🙂
Have you tried to use an incognito tab, clear the cookies of your browser and use another browser or device?
When this doesn't work, you should try to use another internet connection (e.g. cellular data instead of WiFi).
Have a nice day!