Not to be a contrary, but simply inverting the grayscale image will not give you a very good reproduction. I found this out when printing a white on navy job. It's really difficult to look at an inverted grayscale image on your computer and visualize how it's going to look on the shirt when printed with white ink. What I found works well is to first, make a clipping path or create a mask for the background OR the part of the photo you want to print (unless you're printing the square photo with subject, background and everything). Change the grayscale photo to multichannel mode. Invert the resulting black channel, then double click it in the palette, change the color to white in the color picker and the opacity to 100%, and rename it White Ink. Take the mask or clipping path you made and get rid of the background around the image you want to print. Make a new channel, calling it Black Shirt, and making the color black at 100%. Then fill that channel with black. Move the Black Shirt Channel to the top, with the White Ink channel below it. Now, while in the White Ink channel, use the Levels tool under Image/Adjustments to modify the image for the look you want.
I found that simply inverting a grayscale photo gave me a print with very little contrast and too much white ink in all the wrong place. It happened to be a photo of a black kid, which added the complexity of getting decent skin tones of a dark-complexioned subject. The version where I just inverted the grayscale photo made the kid look like a black kid in whiteface. When I did it as a multi-channel file with the white ink channel on a black shirt channel, I could see how it would look modifying the levels in the white ink channel instead of guessing what an inverted photo would look like printed with white ink on a black shirt.