You may find this hard to believe, but new groups have been formed this week (from 2015.06.06 to 2015.06.13):
Choose Your Own Equestria Adventure (CYOEA)
Bluecatcinema's Next Generation Group
Werewolves Of Equestria
AppleFamily Library
Purple Roslyn Fan Club
Foal Free News
Princess Luna Lovers
Christopher Lee
東海岸: Off Topic
Fuck Rules!
The New Equestrian Critics Society
Sparijack
Muppet/jim henson fan club
Jurassic Bronies
Luna/NMM Is Twilight's mother
The Equestrian Borderlands Fan Group
Lawlee and Shawta Presents (nsfw)
Writers helping and checking group
Our first fanfics (nsfw)
Big Bad Wind Pony Fanfics
ghost rider
Earth without FiM
Hippocampus
Fluttercord is Magic
LunaCord-Stories
The Society of Depressed Luna Fans
MLP: AU is Magic
A Circle of helpful friends
WTF (nsfw)
Group for Authors who put their own Stories on their Read-it-Later-Shelf
Grammaire Atroce
rosario pony
Shipping Dock
Moonman is Life (nsfw)
Summary helpers
Female Main Character
The Short Story group
Our lord Techpriest eh!
Thieves in Equestria
TV Tropers Unite!
ponekeks (nsfw)
post anything (that's appropriate)
Chrisverse
discord gem
siren group (nsfw)
siren group
The Dark Side Of Equestria
ScootaBelle Fanclub
The 'Emerald Fires' four part fan-fiction series
The Wasteland (Mad Max/Judge Dredd/Wasteland survival stories) (nsfw)
Elite Society of Disney Villains (E.S.O.D.V)
The Cookie Crusaders
Tirek x Discord
SonicDash
Anime is cancer (nsfw)
ShiningDash

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I personally recommend 東海岸: Off Topic. :D