Bacchae used to be mortal human beings, who were bitten and turned by existing Bacchae. It is unknown how the first Bacchae was made, or if there are any sort of sire/childe bloodline relationships with Bacchae like there are often in vampire mythology. Bacchae live to serve Bacchus, and obey any command he gives. Eurydice is the only know exception to this: she was able to defy Bacchus's orders while still a Bacchae.
The Bite: Victims of the mark of the Bacchae have two puncture wounds made by the long fangs of the Bacchae. Although the exact cause of the transformation is unconfirmed, it is safe to assume that Bacchae fangs may work similar to snake fangs, injecting a venom into the victim's blood stream that takes effect when the sun sets. This venom may also include some kind of soothing/paralyzing agent, since Lilith did not seem alarmed or in pain when she is bitten. It is unknown whether they even drink blood, and if so, if they do so for enjoyment or sustenance, like vampires, or if they survive off of normal mortal food.
The Transformation: Unlike vampires, the victim does not have to die to be transformed - no matter what time of day they were bitten, they are transformed when the sun sets. Victims will start to feel woozy and faint as the transformation approaches, and their eyes will turn yellow as the transformation begins. However, once fully transformed, the body may be considered deceased, as Cheiron called Bacchae "The Undead" - (1.20 - Fame).
Saving Oneself: First, the mark can be erased and the victim saved if the mark is washed with water from the gilded spring before the sun sets the same day the victim was bitten. Second, Bacchus himself has the ability to make a specific fully-turned Bacchae human again (the method is unknown, but he managed to make Eurydice mortal again). Third, if Bacchus is impaled with a dryad bone, he will lose control of (all?) of his followers, and they will turn back into mortal human beings.
Physical Changes: Although paler than mortals and possessing oddly shaped eyebrows, Bacchae can easily pass as humans, mixing and passing through the human population without being noticed. When they change to 'gameface', there are several changes in their appearance: Their hair is frizzy and wilder, their makeup is more pronounced and darker, and of course, their teeth sharpen into two long fangs that they use to bite their victims. Also, their eyes turn yellow and their fingernails grow into long claws. They can change from the human mask to gameface in the blink of an eye.
Powers: As far as we know, all Bacchae have the power to convert a victim with a single bite. In moments of extreme excitement, Bacchae can transform into wolves. It is unknown whether Bacchae are gifted with any sort of super-healing or un-aging abilities, but they are able to be killed. There also seems to be some kind of empathetic bond between Bacchus and his followers - when he is hurt by the waters of the gilded spring, the Bacchae cry out in pain. It may work in reverse as well - the Bacchae seem to be the most mellow and happy when Bacchus is in a good mood.
Benefits: There aren't really any benefits to being sucked into a sick little cult, but it's not all bad - there is plenty of good food and music, and a somewhat comfortable cavern to call home, so it might be a step up for someone living in the streets and begging for food. It is unknown whether Bacchae are gifted with any super-healing, but if they were, a person who is dying might be tempted to join the cult in order to keep living. When not following Bacchus's various orders, the life of a Bacchae seems to also have a fair share of hedonistic lounging around mixed with all-out dance crazy partying, which could be appealing to some.
Consequences: It is unknown if Bacchae require an invitation to enter a person's house, but doing so would explain why the Bacchae are unable to get past the somewhat flimsy Academy Gate (1.19 - The Lure of the Lyre), and why Bacchus lures victims to him instead of just sending his Bacchae out to snag people in their beds. Also, In addition to becoming a mindless slave to Bacchus for the rest of your life, you also lose control of your afterlife. Although the iron-clad rule may have changed post-"A Lady in Hades", the law was that all Bacchae are condemned to Tartarus. Distinction was not made as to whether they actually committed any evil deeds, or whether they were tricked or turned into a Bacchae against their will. Even Eurydice, who died after being transformed back into a mortal human being, was still sentenced to Tartarus. Which raises the issue that all the people who were transformed back into humans at the end of Fame (and who didn't get re-turned by the time Lyre, Liar rolled around) would still be sentenced to Tartarus when they died, no matter how good they lived the rest of their human lives (unless some drastic changes to the Underworld sentencing system actually went through).