Track PIWIK users (no replies)

I would like to know when and how often PIWIK users logged in and had a look at the statistics. Is there a plugin or a way to do this?
If not: it would be more than usefull since you might give users access to verify the frequency of your site and want them to advertise on your site. I would like to know if PIWIK users logged in and had a look.

Tx and best regards,
Peter