Intranet question: visits x visitors

Well, my wordpress installation is located in a web server where many users access inside an local network. It’s a restricted wordpress with an intranet theme.

i think my wp-statistics plugin doesn’t give me correct results about visitors. It is not a bug since everyboy access the website with the same:
– IP
– Browser version (in many cases)
– OS (in many cases)
BUT… they uses different login accounts.

How can i get the number of visitors based on login account and not based on IP + USER_AGENT? Any tips?