This is a very popular effect and has been asked a lot lately on multiple forums. The basic concensus is that the slow person needs to be shot seperately from the fast people, but they should be filmed in the same location (to match lighting etc.). This can be done by holding a green screen behind the slow person (otherwise you will need to rotoscope them). Then shoot the fast people.
In AE, isolate your slow person and put it over the fast people who are sped up using any of several methods.
You could also rent a motion control camera
Well how would I do the blur just on the people? and i shot the slow person seperate, but I just masked him out. I just need help on the fast people,
> Well how would I do the blur just on the people?
Make a matte that defines the people. Use that matte on an adjustment layer; or apply the blur to a layer that just includes the people.
This is a very basic compositing question, and it indicates that you might be better off beginning from the beginning:
Do you mean you want the fast-moving people to be blurry?
You could use echo or some of the other time effects. (You'll need to toy with them a while they're not necessarily intuitive.
You could shoot at a slower shutter speed then speed up your footage.
You could try speeding up your footage and using some sort of frame blending or CC Force Motion Blur...
I highly suggest you follow Todd's link, it's very useful for people starting out in AE and prevents a lot of frustration later on.