Okay, I assume you are talking about a tank for female bettas because there is absolutely NO chance of keeping multiple males in that tank unless you use a divider. If you did use a divider, you could keep two males in it provided the tank is the right shape (ie not shaped so that the fish have an awkward or unusable space if you divide.) To keep two males together in a tank where they are not physically separated the minimum would have to be a tank about the size of a small room.
Female bettas are territorial and aggressive, just not as much so as the males. The general rule when building a sorority tank is to provide hiding places - loads of them. Plant heavily and make lots of caves and quiet corners. If there are plenty of hiding spots, Lady ***** (trust me, you'll probably get one) won't be able to drive the weakest female out of all of them. The other rule is 'the more the better' meaning that if you do get a bitchy fish, she will spread her aggression over all the others and no one particular fish will become unmanageably stressed. Note that if you put in a female that is weak or sick or a lot smaller than the others, it is not uncommon for several or all the others to band together to attack one. If this happens there is no other solution than removing the target fish and housing her alone.
With some territorial species it is recommended to add them all at once so that no fish is encroaching on another's territory when first added (a guaranteed way to spark aggression). This is not necessary with a tank for female bettas. They tend to claim hiding spots rather than swimming space. You can fit six in a tank that size. It's not necessary to put them all in at the same time but you should put the first three (preferably four) in together. This avoids one fish being 'picked on' too much.
Your tank is roughly 8.5 gallons for the americans on here, so if your filtration is good you COULD put some otos or corys in there. Just avoid shoaling midwater species with females, which may snack on their fins.
Finally, NEVER keep males with females. If you accidentally get a plakat (short finned) male, remove him as soon as you recognise him. Do not expect the sorority tank to be peaceful - there will be fin nipping and it's possible that you will have to remove and rehome either a bully or a target. And good luck!
