Best technology to have a specific person's proximity in a room turn on a light outside the room
Since it seems allowable for the person to wear some kind of tag (you mention RFID), I'd probably start by looking into IR.  Tags would wake up every few seconds and send a short message via IR.  These would be on/off modulations of something like a 40 kHz carrier.  That allows use of sensitive and cheap IR receivers, like the Vishay TSOP series for example.

Keep the messages short, but with enough of a checksum so that random noise or message collisions are very unlikely to look like real data.

Each tag is transmitting for such a short time that collisions are unlikely.  Even when they do occur, clock drift between tags will make collisions of successive messages unlikely.  This is a good reason NOT to use a crystal.  This is a easy job for even a tiny microcontroller.  I've done more complicated things with a PIC 10F202.  The PIC can clock out the 40 kHz carrier pulses directly.