I added two members (Jon and Maggie) to two groups I created on our website (https://scarlettscampsite.org.uk/), but when I tried to add a third (Dick) I couldn't add him. If I open either of the groups, Dick isn't in the list. And he disappears from the list of approved members. If I click back in the browser, he's there in the approved list, but if I try to add him to the groups, it doesn't work and he disappears from the approved list again. I've tried logging out of the site and back in but it makes no difference. What's going on?