95:9 One may prepare a single eiruv for several people so long as it includes enough food for two meals for all of them. One must give it to them through another person as we do for eiruvei chatzeiros (see 94:6). One may not prepare eiruvei techumin for someone without his knowledge. The one who places the eiruv, if he's acting as agent for all of them, says, "...it will be allowed for (first person) and for (second person)...." If he's also placing it for himself, he says, "...for me, for (first person), and for (second person)..."
95:10 The eiruv must be deposited in a place where it would possible to eat it at twilight without doing anything that the Torah prohibits doing on Shabbos (as opposed to something rabbinically-prohibited). Therefore, if one placed the eiruv in a pit and covered it with dirt, the eiruv is not valid. If he covered it with a stone, the eiruv is valid. If he placed it on a rigid tree branch, the eiruv is valid but if he placed it on a soft branch or reed, the eiruv is invalid.