Ok this has been 10 days with very little response and is indeed a realistic prospect that this could happen to you. I can see that this really would be one of those "OMGWTF DO I DO" moments so I will tell you what I think you should be thinking about and what your priorities are. In essence this is one of those very high risk moments that you need to be switched on.
Primarily - You were sent to a noisy party that is full of people not a stabbing and yet you have found one- No one has reported it yet. It's very likely VERY recent. This means your offender who is likely armed is still VERY nearby.
Your priorities in order of importance are:-
1) Protecting the public - Your biggest threat right now is your stabbed/possibly dying victim.
2)Protecting yourself so you can achieve objective 1
3) Locating the offender so you can achieve objective 1 in the short term.
4)Gathering evidence that would lead to the successful prosecution of offenders to achieve objective 1 in the long term.
I hope it will be patently clear that you can't do much alone. Your actions in order to achieve your objectives are :-
1) Create a safe space for you and the victim. If people are present who will help you direct that they start administering first aid. You are in a crowd- Position yourself so you cant be attacked from behind if possible. Until you know it's safe stay on your feet-Try not to be the one crouching over the victim doing first aid- You have other things to do.
2)Call up and let them know it is urgent. Give the key pieces of information keep it accurate, brief and clear. I would suggest something like. " AB123 Urgent assistance required, details to follow, over". This will inform FCR and everyone else on the talkgroup they need to give you room and attention-When you are told to go ahead,
"ABC123 I am inside number 1 the street, Sandford and have an unconscious female bleeding heavily from multiple stab wounds to her arms and torso, she is in a serious deteriorating condition, life is at risk. Approximately 100 youths present, offender unidentified but suspected still at scene and potentially armed. Can I have an ambulance, supervisory and sufficient resources to contain witnesses and potential suspect, over."
Now there is a lot more that you could say but you have there in a couple of sentences conveyed the dangers, the issues and your requests.
Were you working for me, I do not want you "Calling CID" or "Conducting searches". Your role right here and now- Is keep the girl alive and answer the questions that are about to get bombarded on you. If you get time update control with exactly where in the house you are. Make sure the doors are open so your colleagues and the ambo can get in to you. Hold the fort, keep everyone alive (including you), give the updates asked of you and wait for the PS, the Inspector and everybody they can muster to descend on you.
In brief look at what the most important things are and try to set the wheels in motion to achieve them. Your role in this situation is largely information passer and first aider and bodyguard for everyone around you.