Cannot add new family member

I want to add a new family member to my family group. I currently only have three in the group including myself, so have plenty of space for new members. However, whenever I tap the 'add new member' icon in the top right hand corner of my iphone screen the new pop-up page flashes open but is a blank white screen, and then very quickly closes again without even giving me the option to try and add anyone or to see who I can add. I've attached a screenshot showing the screen that quickly opens and closes again when I click the add new family member button.

It's been doing this for a few weeks now. I've checked software/ios updates are ok, rebooted several times and it still keeps doing it. What could be causing it and how can I add a new family member if it's doing this? Is there another way?

Thanks, Paul.