Please be aware: Memberstack no longer officially supports our integration with Squarespace. It's unlikely we'll have the capacity to provide support, fix bugs, or add features going forward.
There's no way to hide a folder in your navbar, but you can show and hide the pages inside of a folder based on the member's current membership.
To do so, you'll need to copy the /#/ms/content/... links from the memberships page.
In the above example, I added #/ms/content/free to links to free content, and #/ms/content/pro to links to pro content. I also added #/ms/content/!pro to a page I wanted to hide from pro members.