hi

There are two way to do this:
1. Operartor Message
You create a userdefinedOperator Message, where you can add a lot of informations (p.E User, old and new value of change, and so on). But you can not get a trend of the Values, only a list.
here you find a sample how to do:
http://support.automation.siemens.com/WW/view/en/24325381

2. TagLogging
With a script you can read out the @CurrentUser by change and compare it with a internal list. I the internal list you collate to all Users you had, a number (ID) for the User. This number you write in a internal varibale of WinCC. This variable can you archvie with TagLogging. A system-function i do not know.

regards

cumulus