The minimal technical requirement would be adding a files download area that also accepts uploads. This might also increase site bandwidth requirements, particularly if the manuals are direct scans rather than transcriptions. There might also be, for lack of a better term, editorial requirements -- approving uploads (if desired), deciding what's a duplicate, organizing the download area, etc. If I were running the site, I might be concerned about copyright issues -- technically, distributing manuals in this way would constitute copyright violation. That said, I know of sites that do this, and I doubt if reasonable camera manufacturers would mind -- but some bureaucratic lawyer at a camera manufacturer might think otherwise and cause problems.
Originally Posted by gnashings
Another approach that would be simpler and less legally risky might be to add a section with links to other sites with manuals. As you say, most such sites are brand-specific (indeed, some are at the camera manufacturers' own sites), so it would be fairly easy to organize it by brand. The main drawback to this is that these sites' URLs can easily change, so it'd require frequent checks and updates to keep the links current. There'd also be no way for an individual to upload a manual that's not covered in the existing links. In principle, that person could put together his or her own site and then add it to the APUG database, but that's more hassle than just uploading a PDF (or whatever).