Thank you for your help.
I don't understand security well, but it seems that giving the subuser read access to /home/, /home/main/, and /home/main/example.com/ is a risk, so I'd prefer to avoid doing so if I can.
So I thought maybe I could try it the opposite way, putting the actual files in the subuser's directory, and then creating a symbolic link in the main user's directory:
(1) Create /home/subuser/test/, and make sure /home/subuser/ and its contents are chmod 755.
(2) Log in as the main user, and create a symbolic link with: ln -s /home/subuser/test /home/main/example.com/test
When I do that, the main user can open the symbolic link without errors. However, when I try to open http://example.com/test/ in a browser, I get a 403 Forbidden error. My .htaccess file has Options+FollowSymLinks as the very first item. My error log says, "Symbolic link not allowed or link target not accessible." What am I doing wrong?