There is someone on this tank who has a sorority in a 5 gallon, but his are VERY specialised circumstances - his girls are closely related, have lived together for a very long time, and are apparently extremely close friends, being happy with each other but killing off all other females. I would definitely NOT advise doing this.
If you want to start a sorority, follow Pitluvs's advice and get AT LEAST a ten gallon. :) A five gallon really is just asking for trouble, both in terms of space and bioload. In a five gallon, one betta and some shrimp or a snail are really all you want to put in it, unless you divide it for two bettas. :)