Please specify by what way the guests may reach the files of the closed groups? From my side, it works fine - the "Files" tab on the Group view page contains the info about "Private content".
LeonidSAlex T⚜️ The folder structure is still accessible to anyone on the internet. I've created this group on UNA (https://una.io/page/view-group-profile?id=289) and I sat the field 'visible to' (Secret), however, all folders under 'Files' tab is accessible to anyone (although the files inside the folders are not accessible). The 'Files' tab of the group should not show any information under it. it should be similar to other tabs; Posts, Photos etc. this is a serious privacy problem.
Oh I dunno really. I use the UNA for business, and users can upload a files a specially for my own law magazine, and its working fine. No one can see the files if the chose "visible only for me" option. Can you provide more details, I will try to help you.