How can I track which event photos belong to each person or group afterward?

Super-low-tech (or very high tech but we take it for granted: Take a photo of the card either immediately after a photo sequence it relates to and/or ensure that the subject is visible in the background. It's not 'automated' but if the card fills a majority of the frame then the card images will be easy to spot and extract when all photos are viewed in thumbnail view. I make a separate 'emailme' folder per event for these and related photos.

Plus:

I use versions of this with or without prepared cards.
If meeting people who I give my details to I photograph what I give them, possibly with some identifying detail such as date and time. If people give me email or other contact addresses I photograph the details as part of the picture stream, ideally with the owner in the image as well. If there are multiple people who give me details, names etc I write brief notes against each (yellow jacket, long hair, glasses, ...) and have that as part of the photographed image as well.

If people give me a printed card I photo graph it. I may then return it or not. I explain that a year from now I may or may not know where the card is but, if I can find the photos I'll also have the contact details.

If people write down contact details such as email / kype / facebook ids / phone numbers I always get them to confirm with me what is said. I usually spell out tyhe addresses and add laarge letters in my writing next to anything at all suspect. In days gone by I have had apparently crystal clear addresses turn out to be indecipherable.

For events or similar I have a standard handout slip printed in 2 x 8 = 16 to an A4 sheet which I customise with even name and a photosite URL. It also has my contact details and room for a reference number. The reference number is the 4 digit frame number from the camera of 1 photo of relevance. Where I am moving through a crowd and trying to maximise photo time I may hand out slips and advise them that they will be in time order so if they look at around eg 8:37pm they will find their photos. Time could be written instead but frame is superior. Time is quicker verbally and more memorable.

Prior to the event etc I create a boilerplate page "These are not yet the photos you are looking for re ..." whatever, and then point a URL shortner at it. I nowadays use a 2 stage URL shortner. I use bit.ly as the fron end as it gives a user choosable address string. I use the goo.gl shortner (from guess who) to produce a code related to the actual site and I point the bit.ly system at the goo.gl "address". I do this because goo.gl gives superior stats but only a very random address code whereas bit.ly gives a good address but less good stats.

In a pinch I make up goo.gl codes while I am away and hope that they are not already assigned. Whgen I get back I set them up appropriately. As upper and lower case are counted as non identical you can usually have a fair chance of a unique address if you are creative. eg bit.ly/lady_with_small_child is liable to be unique. Alas bit.ly/AnzacDay2012 was not ! :-(. You can allways use eg bit.ly/date_nnn with good hope of success eg bit.ly/290512_101 Bit.ly addresses are also accessible as j.mp addresses
ie bit.ly/NZPHOTOS = = j.mp/NZPHOTOS. (They recently changed the way menus are interpreted and what was an index list with jumps is now a heap. Work needed).

It's all a lot easier than it sounds and quite useful.

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Less formal example.
Ensure addresses are readable.
Note to self: Find better ways to take 'portraits from semi-outside moving train.
[Malaysia].

Nikon toting China National TV photographer.
Chance meeting in a cafe.

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Ugly, garish, unprofessional looking, cheap, quick, easy, effective photo website handout slip. Someday I may make a better looking version, but quite possibly not. I'll let them judge by the photos :-). (Olin says that is not the way to do things :-) ).