7 gals is a bit small for a sorority.. female bettas are aggressive and territorial, and there won't be enough territory in that sized tank for them all to be happy, lol.
I've found 5-6 females to be best to spread out aggression, with good results in 10-15gal very well planted tanks. PLants are very important because if one is being bullied, they can get out of the line of sight of the bully.
Also, when adding new girls, it's a good idea to take out the originals, float them in a bag or something, add the newbies and then release the originals. Helps ease thing a bit