Download structure for user with multiple groups

[lichti]

Hello,
have read the documentation about the user groups settings. However I struggle implementing following case:

I have 4 categories (product1 to product4). Each category contains some files (manuals and software installers) for download. The files are visible only for registered users. The category names are visible for everybody.
For every product I have generate a user group with the same name.

How can a user "own" multiple products and can access their related downloads ?
I do not manage to set get it to work. (Either all downloads of all categories are visible for a user or only the downloads of one category.)

Any help would be welcome.
I would also accept a "this can not be realized with jDownloads" to avoid further investigations.

Thank's a lot in advance.
 Ralf