The site-lisp directory for Emacs is:
/usr/local/share/emacs/site-lisp
So to share this site-lisp directory with Aquamacs:
sudo mkdir /Library/Application\ Support/Emacs
cd /Library/Application\ Support/Emacs
sudo ln -s /usr/local/share/emacs/site-lisp
I used this so that I can use the same version of ESS for Emacs (installed by HomeBrew) and Aquamacs.
