Would these users be on the guest wireless or production wireless? Are the access points stand-alone or connected to a controller?
Most of the interesting traffic from these clients will go in\out your Internet gateway so you could setup a SPAN or mirror port to monitor this. I would recommend packet capture over flow as you will be able to capture device information from HTTP headers.
Hope this helps,
I know Cisco WLC's can export NetFlow with AVC profiles, however the Cisco-Flavor of WLC Netflow does not follow the standard v9 template. So I haven't tested yet if Orion NTA will support it.