I want to substitute barley for 1/2 cup of macaroni in a soup. How much barley should I use?
The pasta in the soup is only there to add substance and calories, so the amount is not really fixed. You can change it according to your preference.
Without any further information about how you like your soup, I'd say go for a volume replacement, and add 1/2 cup of barley. When you pour a spoon of soup into your mouth, it is mostly the volume of the barley which you perceive as "amount" in relation to the volume of the vegetables and liquid. So if you were to start consider densities and try replacing by weight, the final soup will appear to have less starch than the original.
This is only a starting point, if you find out that you don't like the amount, you can always adjust it for the next batch to whatever you like it to be, without fearing interactions with the rest of the soup.
I'd use a weight replacement so the amount of carbohydrates is the same. One pound of elbow macaroni is about 4 cups of dry pasta, so you'd need 2 ounces of barley. 1/2 cup of barley would be about 2.5 ounces, not a lot of difference.
The barley adsorbs a bit more liquid so you may need to add a bit of extra water depending on the amount of soup being cooked.