This is an extension of my guide with the title "Zombie Survival Guide".

You do the same thing as in that guide, but can also choose to make your "camp" larger (if your co-op partners are good). This will extend the camp to the door frame, so that if you are standing with your back to the grenades the door frame will be in front of you.

Have 1 person at the door frame and 1 person near the window (remember to keep the debris on the stairs that lead back to starting room, unless you clear it for survival purposes). Let the other 2 people wander around the same room helping the others. If you have cleared the debris leading back to the starting room, then leave 1 person guarding the stairs and let the remaining (if there is 1 person remaining) person wonder around the same room.

Hint: Don't extend the camp beyond this as it will make the area too large to cover.