This could be done via RSS, email or anything else Paul and Darius can think of.
Then you could chose what type of subscription you would like to sign up to and each user could state what things others can see when they subscribe to them.
eg. I might not want people subscribed to me to be notified of my notifications so I uncheck that box.
Paul and Darius, who I have now decided to call Pop and Daz, please give this a go if you can