Underground groups—subcultures—can be distinguished from independent cultures by their habit of referring constantly to the parent society. Undergrounds by their nature constantly must maintain a membrane of differentiation. Funny/distinctive clothes and hair, specialized jargon, specialized ghettoized areas in cities, different hours of rising, working, sleeping.... The digital underground, which specializes in information, relies very heavily on language to distinguish itself. As can be seen from this list, they make heavy use of parody and mockery. It's revealing to see who they choose to mock.
First, large corporations. We have the Phortune 500, The Chief Executive Officers, Bellcore, IBM Syndicate, SABRE (a computerized reservation service maintained by airlines). The common use of "Inc." is telling—none of these groups are actual corporations, but take clear delight in mimicking them.
Second, governments and police. NASA Elite, NATO Association. "Feds R Us" and "Secret Service" are fine bits of fleering boldness. OSS—the Office of Strategic Services was the forerunner of the CIA.
Third, criminals. Using stigmatizing pejoratives as a perverse badge of honor is a time-honored tactic for subcultures: punks, gangs, delinquents, mafias, pirates, bandits, racketeers.
Specialized orthography, especially the use of "ph" for "f" and "z" for the plural "s," are instant recognition symbols. So is the use of the numeral "0" for the letter "O"—computer-software orthography generally features a slash through the zero, making the distinction obvious.
Some terms are poetically descriptive of computer intrusion: the Stowaways, the Hitchhikers, the PhoneLine Phantoms, Coast-to-Coast. Others are simple bravado and vainglorious puffery. (Note the insistent use of the terms "elite" and "master.") Some terms are blasphemous, some obscene, others merely cryptic—anything to puzzle, offend, confuse, and keep the straights at bay.
Many hacker groups further re-encrypt their names by the use of acronyms: United Technical Underground becomes UTU, Farmers of Doom become FoD, the United SoftWareZ Force becomes, at its own insistence, "TuSwF," and woe to the ignorant rodent who capitalizes the wrong letters.
It should be further recognized that the members of these groups are themselves pseudonymous. If you did, in fact, run across the "PhoneLine Phantoms," you would find them to consist of "Carrier Culprit," "The Executioner," "Black Majik," "Egyptian Lover," "Solid State," and "Mr Icom." "Carrier Culprit" will likely be referred to by his friends as "CC," as in, "I got these dialups from CC of PLP."
It's quite possible that this entire list refers to as few as a thousand people. It is not a complete list of underground groups—there has never been such a list, and there never will be. Groups rise, flourish, decline, share membership, maintain a cloud of wannabes and casual hangers-on. People pass in and out, are ostracized, get bored, are busted by police, or are cornered by telco security and presented with huge bills. Many "underground groups" are software pirates, "warez d00dz," who might break copy protection and pirate programs, but likely wouldn't dare to intrude on a computer-system.
It is hard to estimate the true population of the digital underground. There is constant turnover. Most hackers start young, come and go, then drop out at age 22—the age of college graduation. And a large majority of "hackers" access pirate boards, adopt a handle, swipe software and perhaps abuse a phone-code or two, while never actually joining the elite.