Do all those who will be cooking in the house need to be present when the Eruv Tavshilin is made, or can one person per household make it, and everyone else benefit from it?
The latter. This is based on the principle that זכין לאדם שלא בפניו - you can confer a benefit on someone without their being present. You just have to inform them, before they start doing things on Yom Tov to prepare for Shabbos, that it was done.
(Shulchan Aruch, Orach Chaim 527:9)