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It is the material theme and it comes and goes. I don't get it. It never happens on Firefox but edge and chrome pop up with the registration form when I have an invite-only website. I am going to try to check daily and see if there is something I do differently because today it is working correctly on all three browsers. Very weird.
We would like to confirm that this feature is not available in phpFox now. If you want to have it, please contact us (phpFox team) or any phpFox experts for customization service.
From what I remember the following tables are updated when you added a new user (this was a couple of years ago so hopefully it didn't change...I don't think it did though)
