What is a Beta Reader? A Beta Reader is essentially a test reader who reads the story, novella, novel, etc, and determines if it is a viable addition to whatever genre. In this case, a Beta Reader would need to be someone who enjoys the zombie/apocalyptic genre and is looking for something new and fresh to explore.
To be a Beta for any of the authors on this site you must be willing to review their work and determine if it is marketable. What that means is did the story hold your interest? Was it something relatively new? Did it, in a good way, stand the existing genre on its head and do something new? What you're looking for is something unique that makes you as a reader and potentially other readers, sit up and take notice.
You might be asked to provide an in-depth review of what you liked or didn't like and why.