Is it FDM or FDMEE?
The FDM shared folder is one of the application architecture components.
So each application must have a shared folder assigned. In that folder some files like archive files, logs, scripts, reports ,etc. will be stored. Each location will have one subfolder under inbox folder so users having access to specific location will only have access to its location folder thru FDM. For example if they have their source files in that folder, they can access to them thru FDM in order to import, validate, and export to target EPM app.
Regarding permissions, typically you will provision the FDM and HFM DCOM users (so service accounts). In addition to this you can provision any other FDM user if needed but it's not mandatory.