I am experiencing the same problem (cceri.org/action-teams both the "Join a Community Working Group" and "Join a Feasibility Study Team" lightbox forms). Works fine on desktop, but doesn't display the whole form on Mobile as soon as you click on a field to begin typing. You can't see the bottom field at all (although you can still type into it blindly), and scrolling up and down only moves the main page behind the form. Experiencing this issue on my iPhone XR using Safari and Chrome, although website visitors alerted me to this issue so I do not believe it to be isolated.