Yes, you can keep groups of female betta together in what's called a sorority... There are some guidelines you should follow to be successful, though.
- Provide LOTS of cover, preferably with live plants. Ideally your tank would be dense with plant growth. If that is not possible, stuff it full of silk plants and cave like decor.
- Filter and cycle the tank. A sorority environment is stressful enough without hih ammonia levels.
- Quarantine all new fish for two weeks minimum.
- Do not stock any less then four girls, with five or six being safer.