how can i check who connect my system from network and copy/delete any file?

hello every one!

i want know how can i check my all activity log, like who use my computer though network and delete or add files in my system!

in which Organisation i am working in, there is a lot of macs connected to each other they all know user name and password of all machines, they work through file sharing many times it happens that anyone delete file from network and we could not recognised who did this!

kindly give me any solution for this problem.

thanks,

Sajid