I have 4 females in a sorority and just added 2 more (one of which I think might actually be a male, but I'm not sure and so far, so good). They seem to be getting along well. They do mostly ignore each other but will sometimes chase each other around. After the first few days I don't think this is aggressive chasing, one will chase another then their roles will reverse. It is a matter of the particular betta's personality though. I have had one who was too aggressive for a sorority. Otherwise, they do seem to tolerate each other fairly well as long as they have lots of hiding places and I also have a lot of natural plants in the tank so they have a lot of places to go.
The first few hours after introducing them you do need to watch the tank almost continuously I would say. They may seem fine the first 30 to 60 min but have problems after that, so watch them. The next few days the tank needs to be watched fairly closely as well. After that, I do keep an eye on them, they are in my living room so it's pretty easy to keep an eye on them and at least once (usually several times) a day I check them all closely for injuries/ripped fins. So far all have been doing really well.