I'm pretty confident a 'member' is just a subscriber. So as long as the email you enter the comp with, is subscribed to the Stack email newsletter, you're a 'member'.
Hope that helps!
From the T&Cs:
Sign-up
Entrants must have subscribed to receive STACK‘s newsletter for their entry to be valid. Existing subscribers and new subscribers entries will all be valid. Please note that sharing the post on social media is suggested, but not required.
How do you become a member? Is this like the goodguys concierge?