How can I view reports of a single register or a single person within a register?

You can view a report for a single register by clicking the name of that report in the Reports menu in the Site Diary tool. 

When viewing a Site Diary report, there will be filters in the right pane that you can use to select the start and end date, as well as companies and/or users you want included in the report.

You can also export the data as a PDF or CSV by clicking on the respective button at the top right section of the page.