When it comes to males, it is definitely not recommended to keep them together. They will either kill each other right away for territory, or they will lead very stressed out lives.
Females on the other hand are totally capable of co-existing. They can be very aggressive, but if you keep them in groups of 4 or more, the aggression is spread out and they should be able to live together in a minimum of 10 gallons, with lots of plants and hiding places. I have a female betta fish sorority myself. They fought like hell for maybe the first 24 hours, now they're all for the most part peaceful with eachother... except for Daiquiri... my little dictator... had to remove her for a bit just because she was just that aggressive. Drama. lol... I could probably link you to a couple of helpful forums if you were thinking of starting one up if you're interested!
At first I had integrated a few females into my 20gal (see my tanks) community tank. After a while, I fell in love with the fish. I wound up taking all my community inhabitants to the petshop (except my eels) and buying more females. My sorority was awesome.
Then I got my 125 set up. I lost one eel and about ten female bettas after the first week or so. I was pretty crushed. The 125 is a uniquarium, so I couldn't do anything to minimize the water flow, and I had to add powerheads to keep the water looking clean. I thought I had to give up on my bettas.
I noticed though that the ones that survived were very healthy. All of my betta stock is PetCo bought- and I've noticed that a LOT of their bettas arrive sick or just weak. A few months later I'd find out that they get their bettas from a single breeder, and I talked with the breeder and found out that only the dregs of his spawns get sold to PetCo.
I've added about five or so females to my 125, and they're all doing well. Picking them out is very difficult however, and I did have more than one die on me in the process.
I've spent months watching the females and how they interact with one another. After the first spats over territory- they stop. My tank is moderately planted, but I have a HUGE rock pile with holes throughout. I have rocks in there that you can't even see from the outside, but I knew that my bettas would appreciate the space, so I spared no expense.
I have three red females that have been in the tank at least two months, and they actually school together. It's so neat.
Recently I added three males. That makes ten females, and three males in the same tank. I'll post some more recent pictures soon, as I've just noticed that I don't have any more recent ones.
Of course, these are not the only fish in the tank. I've got a very explicit strategy for picking the fish and setting up the tank for multiples if anyone here is interested in talking/ trying it.
I really feel like my process works, but I won't know unless other people try it and are successful too.