exam in few dayss..pl solve this

[gmat20092009]

Studies show that young people with higher-than-average blood pressure,which...

my question is we know that the subject is young people and anythn which comes with (with,in addition etc except and ) makes the verb singular.
Now can WHICH refers to young people....cos we know that the subject is young people and with is just an addition..

thanks

[vaibhav.iit2002]

The rule you stated is 100% correct but it is not applicable here. It is applicable in sentence such as given below:
Ron with me, is .....

In your sentence, with is not used in similar context.
I hope you got it.