Sounds like a good case for NFC. Put RFIDs in the attendee badges, and a sufficiently-advanced camera app could auto-tag those people as the picture is taken.
Does that sort of technology work directionally? Typically the place was so crowded that you'd often have 40 other people within 15 feet when taking a photo, so it'd have to identify exactly the people in front of the camera, and not to your sides or behind you.
GP conflated RFID and NFC a bit, at least their colloquially accepted meanings. What we normally call 'NFC' has a range of <20cm, often <<20cm (http://en.wikipedia.org/wiki/Near_field_communication). This is what also makes NFC more suitable for mobile payments than RFID.
Hmm; true. Maybe just passes with large, simple QR codes on the front of them, then? Then the device taking the picture wouldn't even have to decode them just-in-time; the data would be encapsulated in the picture itself.
RFID refers to a family of technologies that operate over a variety of frequencies and given by the ISO standards. Specifically, 134kHz, 13.56MHz, ~915MHz, and 2.4GHz. The 13.56MHz standardized frequency for RFID is the same as that for NFC. The RFID ISO standards that specify the use of 13.56MHz are ISO 14443 and ISO 15693. NFC is specified in 2 ISO standards, but the more relevant to the discussion is ISO 18092 and it is derived from ISO 14443. So, there is some interoperability between ISO 14443 and ISO 18092, and therefore there is good reason to "conflate" NFC and RFID.
EDIT: I should cleanup and redact the Product Requirements Document I wrote 5 years ago for a transceiver board we built that could talk ISO 14443 and ISO 18092 (as well as ISO 15693) using a single transceiver ASIC and antenna tuned for 13.56MHz.
