Ok from what I remember (I searched for tips on this as well).
- Agree to leave all your stuff and follow the guardian inside.
- Watch the initiation etc., and agree to kill the sacrifice/captive.
- When you have the dagger, go upstairs on the ledge on the left side (facing the captive). The.. uhm.. head Mythic Dawn woman will follow you up, but not the others.
- Kill her with the dagger. Try sneaking to get the 6x bonus. For some reason she'll be facing one direction no matter where you are. You can also sneak and steal one of your items from her, and she won't mind. She'll say something like "Pfff... that's worthless to me anyway". But the next time you try to steal she'll attack, so choose wisely if you do this approach.
- When she's dead, you can collect all your stuff from her, as she's the one carrying it. I think she also has the key you need. The others will still not respond or follow when you kill her. This is probably a bug, but...
- Now you have all your stuff. Go back down and kill the others, then explore the entire cave and kill everyone.

Not sure if you have to do this, but I did to ensure that the captive could escape safely. You'll also find some nice loot, I think.
- Go back to the captive and release him (or was it a her?). He/she will start running off. talk to him/her, and he/she will thank you and tell you where he/she can be found later to give you some free training.
You're done.

Edit: And remember to grab the book before you leave!

But don't take it before you kill that first woman.