How to find User KPI Report and what it is used for
The User KPI Report is a per User report to show which users touched an invoice. You can use it in multiple different ways to see how many invoices a specific user has touched or which vendors are being touched by which user etc.
To get to the User KPI Report, hover over 'Reports' and select 'User KPI Report' as shown below. 
This bring you to the 'User KPI Report' Screen. Here you can see all the different ways you can search for the users that are interacting with each invoice.
Below are a few examples of how the report works using a couple of different search criteria. The first one below is just searching for all users. Here you can see how many invoices each user has touched.
The second example is searching by 'Invoice Number' and 'Activity Start Date' you can see this invoice was worked on by two different users.
There are many different ways this report can be utilized to gather important information on which users are working which invoices and or how much a specific invoice is being passed between different users!