I don't know if this information is on the wiki, but as far as I know all users under a given account are in the same group - so as long as your SpamAssassin installation is executable by others in the same group you should be ok. You'd just have to use the full path when executing spamassassin:
ex. Install Spamassassin as user1 (who is in group1)
As user2 (who is also in group1), run /home/user1/whereever-spamassassin-is-installed/spamassassin
Each user should then be able to have their own settings which should be controlled by the .spamassassin directories in their respective home directories.
I hope this is reasonably clear -- I'm still in the process of evaluating Dreamhost, their email service seems to fluctuate constantly between quite good and HORRIBLE. But I'm slowly experimenting with getting a SpamAssassin setup similar to this working on my account and in theory this general process should work.
Good luck! If I do go with this and get it to work I'll post the steps on the wiki.