There are no real covens of vampires, old or new, since there are no vampires- old or new. Sorry, also applies to unicorns, werewolves and elves.
A leader of a coven in London it's on page 331-332 in twilight
The vampire from the Denali coven that liked Edward was named Tanya.
The Amazon coven The Denali Coven The Egyptian Coven The Irish Coven The Olympic Coven The Romanian Coven The Volturi Coven
You can join a vampire coven in the coldest and rainiest place on Earth, except for the North and South pole
the Alaskan vegatarian vampires are the Denali clan.
Nahuei
Ireland. (how practical of them)
Carlisle's coven are the Olympic coven. It tells you in the vampire index at the end of breaking dawn.
There are many vampire covens online. They all claim to be real. Some examples would be the blood coven and vamp coven.
Heidi is a vampire. I believe she is apart of the Volturi Coven.
Including the Cullens, there are seven different vampire covens. The Amazon Coven, the Denali Coven, the Egyptian Coven, the Irish Coven, the Olympic Coven (The Cullens), the Romanian Coven and the Volturi Coven. There is also the Volturi Guard, the American Nomads, and the European Nomads.
The Italian coven of vampires in Twilight are called the Volturi. They are an ancient coven, almost like royalty in the vampire world.