I guess it depends on who you ask. There’s a lot of debate on this topic. From what I understand, female captive bred bettas are just as solitary as males and sorority tanks put unnecessary stress on them. I’m told that wild bettas can be housed together though, if you can find them for sale, but should avoid housing pet store type bettas together. There’s a lot of various schooling fish that could comfortably and safely be housed in a tank that size, though, so whether or not a sorority tank can be forced, I’m not sure I really see the point of putting the animals in that position.
But I don’t have any personal experience. So, of course, anyone with betta and betta sorority experience is more than welcome to comment! And, as always, no matter your opinion, research, or experience, please keep comments respectful and polite.
Sororities don’t work. They can be temporarily kept together, but the dynamic will eventually shift and violently collapse. It’s extremely stressful in the meantime, as well, a delicate balance of power that’s more like countries’ “we both have nukes so we won’t fight each other because we’d both get hurt” truces than actual peace.
Betta splendens, the bettas you see for sale in cups, were originally bred to fight. The ones you see for sale most commonly are a domesticated species that are highly aggressive and extremely solitary. They cannot be kept together long-term.
Try wild bettas. B. albimarginata are some of my favorites, available captive-bred but not domesticated. They should be kept in groups, with more males than females, as they’re mouthbrooders and males should be able to take breaks between brooding mouthfuls of eggs. You could keep probably 3 males and 2 females in a 20g with plenty of hiding spots. They’ll flare and circle at each other now and then in showoff fake-fights, but don’t actually fight each other like B. splendens will.
Wild-type B. splendens may also be a possibility, and there are lots of others that aren’t on the top of my head at the moment. You can look on Aquabid to find wild betta species available for sale, though you should always aim for captive-bred ones.