The following is a very brief recount of Artemis and her followers, as told by Robert Graves in The Greek Myths.
The seven sisters were the daughters of Atlas and Pleione, born in Arcadia, on Mount Cyllene.
At nine years of age the seven became followers of Artemis, the Greek goddess (also known as Diana in Roman mythology).
Artemis was the daughter of Zeus and Leto, and the sister of Apollo. She was both the goddess of the moon and of hunting - in particular hunting the stag. She was also the ruler of the stars but later Zeus took on this responsibility.
For a time Artemis and the great hunter Orion were hunting partners. The story of Orion is given on that constellation's page.
Before his demise Orion had the annoying habit of trying to seduce the seven sisters every opportunity that arose.
Eventually, to put a stop to Orion's amourous flirtations, the gods took pity on the seven sisters and whisked them away to the safety of the heavens, where they would reign in perpetuity a safe distance from Orion.