Concerns About Folder Permissions and Security in Gamma for Large Organizations

Sorry I haven't searched here but we have been recommending Gamma to clients and a few have signed up. The problem some bigger clients have is that anyone can join any folder. They want to set permissions to folders for their users of bigger orgs as adding everyone individually is a pain and doesn't organize well. On the flip side its not secure to use folders internally because then other members can just join the folder and see the gamma in it. Unless I am missing something. I tested it this out a number of ways and that is how it has worked for me.