help setting folder permission

Just guessing, but you may need to resort to the command line.

For example:

chmod +a "group:_www allow list,search,add_file,delete_child" /path/to/the/dropbox

Obviously you'll need to decide on which of the (13?) types of access to specify (and possibly some sort of inheritance too), as well as the proper pathname. Maybe some apache guru can shed more light.

Edit: yah, that seems to work. And —once that _www group is aded (via running the chmod command above) —you can then go back to the Get Info window and tweak the settings from there (if Finder feels more comfortable).