SCREEN FLICKER - UNABLE TO JOIN SOLUTION
After restarting the beta, my xbox, and redownloading the beta, etc… I was still experiencing the screen flicker and unable to join error. I then began playing around with my network and discovered a temporary solution to the flickering screen, flashing screen, and/or unable to join error.
1. Turn off Xbox One entirely, unplug from back of Xbox to be sure.
2. Unplug your cable modem and your router.
3. Wait 15-30 seconds then plug in the modem first, router second, and once your internet is active fire up the Xbox One.
4. Go to Halo 5 Guardians Beta and join matchmaking.
NOTE: I have a dedicated IP address for the Xbox with all appropriate ports open, NAT is OPEN. Additionally this solution does not fix the problem indefinitely, it will only work for one session until you back out of matchmaking or quit the beta, in order to rejoin matchmaking you will have to follow the steps above again.
Hopefully the devs see this thread and fix!