mNNNNNNN email accounts don't have home directories that you can access. If you want to use a custom install of spamassassin, you have to create a user with shell access. If the user name is "myuser" then ~myuser can be used to access that user's home directory.
But if you originally install SA in user1's home directory, and want to share that installation with user2, you probably will have to login as user2 and then link to user1's files. (This is because of the way the permissions work.)
I started using SA before the nice set of instructions was published, so I don't have the same setup. It looks like that if you follow the instructions you ends up with 2 directories in user1's home: sausr and saetc. So to share the installation with user2, log in as user2 and do:
ln -sf ~user1/sausr .
ln -sf ~user1/saetc .
This will make a symbolic link from user1's SA code to user2's home using the same names. (That's a period at the end of the command; it means "put the sym link in the currect directory, using the same name as the original".)
This should replace steps 1-6 of the instructions (ie, downloading and building SA). It may be possible to replace some of the other steps with symbolic links, if you wanted to have all users share the same spam rules, etc. Otherwise, you can just continue with step 7.
As I said, I rolled my own SA installation before the instructions were published, so I haven't tried this myself. I would experiment with this on a non-critical user and make sure mail is being handled properly before applying it to other users.