From the article "The Challenge of Zork, Electronic Games Magazine":

"...To test the new, slimmed down version, Marc, Dave and Joel enlisted the help of their former M.I.T. roomie, Mike Dornbrook. Despite the fact that Mike worked at M.I.T.'s Lab for Computer Science, he had never played any video or computer game. That made him the perfect guinea pig. (Mike has yet to drop a quarter in an arcade machine.) As the origianl Infocom play-tester, Mike found himself fielding frantic letters and phone calls from harried Zork players who had come across insurmountable problems. Although he concientiously answered each and every inquiry, it got a little boring to answer the same 30 or so questions again and again. Even establishing an Infocom hint service, which gave out sage advice for $2 a question, wasn't enough.

"To compound Mike's dilemma, graduation time from M.I.T. rolled around, and it was time for him to move off to the University of Chicago's business administration program. Uneasy about abandoning Zork players completely, Mike tried to convince the others to hire a replacement for him. Impossible, he was told. After all, Infocom was a fledgling company, and there was already too much to do in the areas of production, distribution, and design for Zork's sequels. How could they hire a full-time staffer just to answer the mail?

"Dornbrook countered with a compromise: If Infocom would let him borrow its trademark and establish the Zork User's Group, everyone would benefit. His original intention was to operate the business out of his Chicago dorm room, but it proved easier to wax his father out of retirement and set up an office in nearby Milwaukee.

"The Zork User's Group had a humble beginning. The junior and senior Dornbrooks catered to a small but faithful following, providing almost the only support available to any computer adventurers. The company devised and sold complete maps for Zork I and its sequels, sent out new game announcements and insider's information, and answered an increasing volume of clue inquiries. In 1982, after Infocom had begun enclosing information on the Zork User's Group in each game package, the player questions finally grew too numerous to handle. Mike realized he needed a more efficient means of helping Zorkies in need. After experimenting with sealed envelope kits, scrape-off clues made like instant lottery tickets, and a number of other ideas, Mike was stumped. At a party, a friend suggested using invisible ink, which could be made visible by running a special developing pen over the hidden answers. Mike loved the idea and immediately tried to get started on it - only to find a major obstacle in his path: Where to find a company to produce the books? It turned out there are only two manufacturers in the U.S. capable of printing up "latent image process" books, a fact Mike discovered after exercising the same sort of perserverance that helps him slove adventure games. Luckily, one of the printers was nearby.

"The hint booklets, called "Invisiclues", are now available for all three Zork adventures. Their apperance caused a minor sensation and have contibuted to the games' popularity. Gamers used to writing software companies, or simply shelving their adventure games when stumped, no longer have to tear their hair indefinitely. The answers were there for those who wanted to find them, but hidden well enough for those who didn't. Each book was packaged with a developing marker, which the gamer passes over a particular section to answer specific questions printed in the booklet. To make things a little less obvious, dummy questions are liberally sprinkled in to keep players from learning about still-undiscovered areas just by reading all the questions.

"The Invisiclues concept took off like a shot, and late last year Mike Dornbrook joined the full-time staff of Infocom. Although officially, the Zork User's Group has been dissolved, would-be Zorkies need not worry - Mike is overseeing Infocom's takeover of all User's Group functions, expanding clue and map support to cover all of Infocom's releases. As it stands now, each time a gamer buys an Infocom game, he or she finds a coupon insinde. With the coupon, an Invisiclue booklet and complete map can be bought for $4.95 (what it costs Infocom). Without the piece of paper, though, the set goes for $8.95, a move Infocom hopes will discourage software pirates. With Infocom's money and resources behind the clue booklets, the packages and maps are being redesigned (in look only - the content remains the same) to be consistent with the company's other high-quality accessories. A customer newsletter is also on the horizon, with its format derive from Mike Dornbrook's "New Zork Times". Posters, T-shirts, and other Zork-related souvenirs are also planned."