From the White Wolf Wiki, the most complete White Wolf reference

In Vampire: The Masquerade, Les Amies Noir, also called the Friends of the Night, are the secret society that largely governs clan Lasombra. The Friends of the Night are an initiatory society: members are invited at the behest of existing members, and candidates are tested both formally and informally before being invited. A vampire is considered for membership only once, and any candidate who is rejected will never join the society. Every Lasombra except the most dullard amongst the clan knows that the society exists, but the only ones who know significant information about the society are members.

The Friends of the Night ensure that the clan runs smoothly, in accordance with the Lasombra predilection for power from the shadows, the Friends of the Night generally ensure this smooth operation through ruthless behind the scenes manipulation, well placed words in well placed ears and the discrete occasional assassination. The most infamous practice of the Friends is its role in managing diablerie within the clan. The Lasombra believe that if diablerie exists, somebody's going to do it, and they'd rather that it be applied productively - proximity to Caine is an investment, after all, and that investment can be spent wisely, or transferred to a more responsible keeper.