Have your company purchase a Web Filter. Out of the box this will be something that can provide for you. Orion NTA isn't the best answer for Web Site reporting.
One thing you can do is on your Firewall, say if you are using a Cisco ASA is to ensure your logging is set to Information and that syslog 304001 is not disabled. This will then send website accesses as syslogs to the server. I warn you though that this will flood your syslog server and make life a pain. You probably won't be able to have much more insight beyond 30 minutes into employee activity this way either.
You cannot do this with NetFlow because NetFlow doesn't capture the data in the HTTP header. You need the actual HTTP header because it is extremely common for web servers to host many sites at the same IP address; simple reverse DNS does not work for this type of tracking.
As mmelton stated, a web filtering appliance is the easiest way to get this data. If you want something free, look at httpry, but realize that with httpry a good bit of customization and scripting will be required to get exactly what you're asking for.