Torrent client for OS X that can be shared between multiple user accounts I am looking to transform my macMini into a media server that is shared by all my flat mates. 

I want to fully share: the torrent client, the media centre application and the media library.

Now the question is what torrent client can be used between several accounts and preferably from Guest account also. If Guest account cannot be used, I will create a generic user instead of goest.

Still, I would strongly prefer a torrent client that is able to run as daemon, without requiring you to login.
You could use Transmission. But instead of just using the app set up the daemon and use the web interface or the remote client GUI to access the service.

This way you will be able to access the service even from other computers without having to open a user session on the computer.