I am trying to make a few shirts for my wife and crew at the local EMS location. White on black. I would like to add a reflective material to the ink for nighttime use. Should I mix it with the ink first, or sprinkle it on after it is printed while still wet? It is glass powder like here.
http://www.amazon.com/Powder-like-Refle ... B008B9VYZ2

There are reflective inks available for screen printing. If you mix anything into an opaque plastisol or water based ink it will likely be covered in ink and do nothing. For what you suggest applying it on top of wet ink would work best but you don't know if it will bond or last or be safe for people. I would use a tested and used ink for this if you need it.

Or reflective heat transfer materials work really well.

I agree with Jonathan, I would go with a reflective ink or heat transfer. I am delving into some heat transfer stuff to expand my business some and there are some good options out there especially for smaller quantities.

I've used reflective ink in the past and would like to add to the conversation that you'll want to use a much lower number mesh count, perhaps a 60, to allow the ink to pass successfully. Those reflective particles wouldn't pass thru a finer count mesh. Maybe ask manufacturer for ideal mesh count.