I had an idea about this project this morning:
What about encoding the different user "guns" with a different IR pulse pattern? Basically, each gun is using a different pulse sequence like various remote controls from different TV manufacturers?
The targets would only need to look for the pulses and be able to decode the signals for X number of players. It would essentially be a rehash of the IR receiver examples that allow you to decode remotes. The TX in the guns would be a rehash of universal remote software.
You still need to tune a lens somehow on the RX,plus you would have a neat easter egg if you happen to program your own gun to be the same as one of your remotes