You want to advertise your game, we agree on that ?
Then, make most of your sneakpeeks and advertisement-focused posts available for everyone, and by that, I mean make it accessible to the public. You want to get people interested before wanting them to open their wallets.
Taking myself as an example (I'm obviously not speaking for everyone out there) : I'm interested to check how your dev is advancing, but I certainly won't PAY for that, nor want to have to register (even if it's free!) to your SS. I just want to check if the way your game's headed, and if what IK see hook me up, I might register and who knows, even sub if it's worth the price !
You can always make a separate sneakpeek post for members only, with more exclusive content in it or whatever.
In short : if you want to advertise your game, make those ads more public. People usually don't what to pay for advertisement afaik.
Read above, I explain more clearly what I think shouldn't be paywalled in that case