When I did it on Broadway, there was a display in the lobby where people could sign up. There were maybe 7 or 8 of us who were interested; a staffer (stage manager?) took us into a little room and did a very quick group interview/rap session. We left the room and a few minutes later she came out and gave the "winners" a little paper form and a plastic "finalist" name tag. (We got to keep the tag, along with the juice box Mitch gave us when we were eliminated from the spelling bee. I got mine signed by Josh Gad, James Monroe Iglehart, and Jennifer Simard after the show.)
If I had any advice, I'd recommend that your husband choose clothing and present himself in a way that, while not too "out there," gives the judges something to make fun of if he's chosen. I assume that's what the staffer was looking for.