There are some really awesome threads about sororities on this forum. Here's a sticky on sororities.
I just bag all of my girls up in their own zip-loc baggies when I'm introducing someone new. It's just easier for me to float a zip-log baggie half full in my tank than try and find cups and then hope they don't spill over. Anyways, they all have to be floating at the same time for at least fifteen minutes. The longer the better. Acclimate them like you would any other tank. Release the least aggressive girl(s) first, and then the most aggressive girl(s) last.