Exact method would depend a bit on your contact-users association. You could write a trigger on contact insert/update and make userEntitity records between the user who made the contact record and the associated users.
Have a look at the chatter cheat sheet: http://developerforce.s3.amazonaws.com/website/pdfs/Chatter-cheatsheet_final.pdf
ps: you may get more response at https://salesforce.stackexchange.com/ , which is specialized on salesforce subjects.