I had 7 females in a 10 gallon tank, heavily planted. No other fish or nonsense in there, just betta girls. I had lots of hiding places. I thought everything was stable, they would squabble a little and then get over it.
Then one day I woke up and had 6 fish. I had no clue which one had done it, it's not like she left "fin prints" at the scene of the crime.
Long, tragic story made short, it was another death every other night or so until I was left with 1, who I assumed to be the ichthiocidal maniac. She died a couple days later, I presume from stress and the injuries she sustained over the course of her rampage.
THAT SAID: My advice would be to not start a sorority. Your mileage will vary, and if you're anything like me, you will ignore this advice.
If you must try this out for yourself, please learn from my mistake and don't try it in a 10 gallon tank. Looking back I think a 20 gallon long might work. The bigger the better. I've also read a lot about "dither fish" to serve as distraction. You want something that does not nip fins, but that is shorter-finned and faster than a betta so it can escape when needed. Maybe like a basic, non-fancy guppy or something.
This is what my sorority looked like before the serial killings began, maybe it will help show you what not to do (looking at the photo, it wasn't as "heavily planted" as I had thought, plants need to go all the way up the water column for this to work):
I doubt I'll ever retry a sorority, but if someone said, gun to my head, that I had to do it, I would get the biggest tank possible (like 120 gallon if I had the room) and I would put only 7 girls in there, all from the same spawn, and probably 3 or 4 small schools of other fish like guppies and rasboras.
Please please please read up on this before you attempt.