As to your second question, that's not suggested because directories are owned by certain users and conflicts can occur. But you can do it. FTP access from the world to your Mac isn't suggested either - MAMP is intended to be used as a localhost sandbox, within a LAN or just on your PC via localhost. Otherwise, security issues galore, not to mention many ISP's close the port, slow performance as upstream rates are often considerably slower than downstream and workarounds required for dynamic IP's.
My question was hot to make FTP users without having to make a whole new Mac OS X account for them on the computer. I figured out how to do this. I used PureFTPd and its great. Its exactly what i needed.