Go to the town of Riften and locate the Black Briar Meadery. Ask the Dark Elf at the counter about the Black Briar women, and choose the "Persuade" option. He will answer your question, and you should receive a level in Speechcraft. Note: You must have Speechcraft at about 25 in order to persuade him, and it may require several attempts to receive a level as your Speechcraft increases. There is no limit to the number of times you may do this.