In "Hardtack and Coffee," Billings writes about barbers being members of the unit who wanted to pick up some spare $$. They may or may not have had experience. To a certain extent, if all people want is a trim, that's pretty easy to do. It's easier to shave someone else than yourself, so if you had a razor, you may have found people desperate enough for a shave. My understanding is that in the pre-war Army, barbering was then as it is now, a contract civilian job.
I used to hang out a shingle on my tent that said "Barber. Shave 12 cents, Haircut 12 cents." A kid at one event turned to his dad and went "Can I have a quarter?" I didn't charge him, but sat him up on the stump in front of the tent, snipped around the back of his head while making commentary - "Oh, well, your hat will cover that until it grows back in," etc. Then I lathered his face. I didn't use the razor at all on him, but used it as a demonstration, punctuated with "Is this what it looks like when your Dad shaves?" Best public interaction I had that season!
I sometimes shave myself and find that it draws a crowd. If I draw blood the crowd gets bigger, and little boys say things like "Is that real blood?" "Look, he's really bleeding." "That's really red."
Pine River Boys, Co I, 7th Wisconsin
"We're... Christians, what read the Bible and foller what it says about lovin' your enemies and carin' for them what despitefully use you -- that is, after you've downed 'em good and hard."
-Si Klegg and His Pard Shorty