Show Custom page instead 404 error page

Hi. Nice plugin 🙂
By default unregistered (not authorized) users see the 404 error page if they trying to open something from Premium area. How to change 404 page to my custom WordPress page (with texts about Premium area and signup form)?
Any hooks? Template redirects?

P. S. Will be great if this option (select the page that should appear when you visiting premium area without rights to see this) will appear in the future. And while I want to change this manually.

https://wordpress.org/plugins/groups/