Do all party members contribute to kill streaks and massacres in Diablo 3?

Do the kills via party members and temporary quest guests contribute to the numbers required by massacre and kill streak?

The one question not answered here How does the massacre bonus work?

Also, do you get the exp of quest guest kills? Do they share in any of that experience?

No.

I have been playing online with a few friends and due to their "power-levelling" techniques, I can hardly keep up with both in speed and damage. Basically, each person controls their own sprees, and even if you assisted in damaging the creature, unless you killed it, then it does not contribute to your own spree.