Darkfriends, also known as Atha'an Shadar (People of the Shadow) among the Seanchan, are humans who have been tempted with promises of immortality and power by the Dark One, or by other Darkfriends, into serving the Shadow. Although the Black Ajah are Darkfriends (as are Dreadlords and even the Forsaken), the vast majority of Darkfriends cannot channel.
The War of Power Edit
Once the Bore was opened, the worst side of human emotions were released by the Dark One. These gave rise to many people swearing their allegiance to the Dark One, who called themselves the Friends of the Dark. In this Age there were far more public figures that were openly declared for the Shadow, as well as active leadership. This increased the numbers of seriously dedicated Darkfriends significantly. These people too were promised immortality and power, should the Shadow win.
In the current Age Edit
Most people today are not as dedicated to the Dark One as they were when the Bore was open, although some are still as zealous today. Many people just want to dabble in something taboo and don't realize that their vows hold them unto death (and sometimes beyond it!). There are enough of them in number to be dangerous, however; a city of one hundred thousand may contain five hundred to one thousand Darkfriends.
It is not made clear how exactly such a vast network of Darkfriends is completely organized. In almost all cases the only thing that any of the Darkfriends have in common with each other is that they worship the Lord of the Grave. From the highest to the lowest there are Darkfriends, although rank in the regular world means nothing - Darkfriends have their own internal hierarchy and a king or Aes Sedai would have to obey the worst beggar if they gave the correct signs. A known sign used by the Black Ajah is performed by placing your hands at your side with your thumb held between your index and middle finger.
Darkfriend councils have been mentioned before, thus indicating some kind of central leadership. Since the Forsaken have been released they have taken control of Darkfriend networks and they now direct their actions. 
The Forsaken may run their own network of Darkfriend who obey only to a specific member of The Chosen. Sometimes Darkfriends have to obey another of The Chosen and change loyalties. Throughout the series Liandrin's Group of Black Sisters is ordered by different members of the Forsaken with different purposes.
To become a Darkfriend you must be noticed by another Darkfriend in most cases. Darkfriends are always on the lookout for new recruits to raise their numbers. A cell of Darkfriends might watch people in public and see how they react, and if they think that someone shows signs of a being a possible candidate they will approach them. Often they kill people who refuse to stay hidden. All that said, if someone wishes to become a Darkfriend they can speak against "The Light" trying to be subtle, although that runs the risk of being caught.
A Darkfriend with great significance is Padan Fain, someone who was turned into a "hound" by the Dark One and gained the ability to seek out Rand al'Thor, Matrim Cauthon, and Perrin Aybara. Moiraine says he is something more that a Darkfriend since his encounter with Mashadar in Shadar Logoth.
Returning to the LightEdit
Verin's case is most interesting because she, and her Warder Tomas, realized early on that they had made a mistake but decided on a much riskier course; they would remain Darkfriends so that she could study the group in depth from within. To circumvent the Great Lord's oath which made her forswear betrayal until the hour of her death, she waited one hour prior to her suicide before betraying him and his followers to Egwene al'Vere by providing the future Amyrlin Seat with ample notes of its operations, goals, and prophecies, including a large, though probably incomplete, list of Aes Sedai who were also Black Ajah.Ingtar allowed a Gray Man to enter Fal Dara during the chaos of a Trolloc raid. The Gray Man tried to assassinate Rand al'Thor but the arrow struck the Amyrlin Seat, Siuan Sanche. During the hunt for the Horn of Valere, Ingtar expresses sadness and regret about the state of the world, how everything wholesome and good was being swept away and forgotten. During the misuse of a Portal Stone on their journey to Falme, Ingtar sees his life played out over and over again. He sees the foul deeds that the Shadow may force him to commit, each act more horrid than the last. He resolves to find the Horn of Valere for his own salvation. His plan ends differently once he realizes the cost to return to the Light. He confesses his sins to the Dragon Reborn before Sheathing the Sword...
"What are you saying?"
"Humankind is being swept away everywhere. Nations fall and vanish. Darkfriends are everywhere, and none of these southlanders seem to notice or care. We fight to hold the Borderlands, to keep them safe in their houses, and every year, despite all we can do, the Blight advances. And these southlanders think Trollocs are myths, and Myrddraal a gleeman's tale. It seemed the only way. We would be destroyed for nothing, defending people who do not even know, or care. It seemed logical. Why should we be destroyed for them, when we could make our own peace? Better the Shadow, I thought, than useless oblivion, like Caralain, or Hardan, or... It seemed so logical, then."
"You aren't making any sense. Say it plain, whatever you mean. You are talking crazy!"
"You are a better man than I. Shepherd or lord, a better man. The prophecy says, 'Let who sounds me think not of glory, but only salvation.' It was my salvation I was thinking of. I would sound the Horn, and lead the heroes of the Ages against Shayol Ghul. Surely that would have been enough to save me. No man can walk so long in the Shadow that he cannot come again to the Light. That is what they say. Surely that would have been enough to wash away what I have been, and done."
"Oh, Light, Ingtar." I think.... I think wanting to is enough. I think all you have to do stop being... one of them."
"Rand, when Verin brought us here with the Portal Stone, I- I lived other lives. Sometimes I held the Horn, but I never sounded it. I tried to escape what I'd become, but I never did. Always there was something else required of me, always something worse than the last, until I was.... You were ready to give it up to save a friend. Think not of glory. Oh, Light, help me. There has to be a price, Rand. There is always a price. Perhaps I can pay it here."
"It is every man's right, Rand, to choose when to Sheathe the Sword. Even one like me. Go, Rand. Take the Horn where it belongs. I always knew the Amyrlin should have given you the charge. But all I ever wanted was to keep Shienar whole, to keep us from being swept away and forgotten."
"I know, Ingtar. The Light shine on you, Lord Ingtar of House Shinowa, and may you shelter in the palm of the Creator's hand. The last embrace of the mother welcomes you home."