Eruvin Chapter 7, Mishnah 7. In the next mishnah we will learn that there is a specific measure and quantity required for shitufei mevo'os (joining of alleyways) and eruvei chatzeiros (joining of courtyards) - how much food there must be. Our mishnah, before reaching that, deals with what happens when the measure is no longer met: there was a proper measure, and afterwards some of the food was lost, spoiled, or something happened to it. What is the law then?
"Nisma'et ha'ochel - mosif u'mezakeh":
The mishnah says: "Nisma'et ha'ochel" - the food that was designated for the eruvei chatzeiros or the shitufei mevo'os was diminished, and it no longer contains the required measure. In such a case - "mosif u'mezakeh":
"Mosif" - he adds to the quantity that is there in order to bring it up to the required measure. Sometimes the addition is made from the food of the residents of the alleyway, and sometimes from the food of the person who is doing this on their behalf.
"U'mezakeh" - if he is using his own food on their behalf, he must perform a zikuy (conferring of ownership) - the same zikuy discussed in the previous mishnah, where he gives it to another person to acquire it on their behalf.
The mishnah adds: "V'ein tzarich l'hodia" - there is no need to inform the residents of the alleyway that he added food and conferred ownership of the additional food upon them, since they already agreed to be part of the shituf, and this is nothing more than refilling the barrel and the like.
The details of the Gemara regarding the law of informing:
The food was diminished but some of it remained: even if he completes the measure with a different type of food - it was wine and he adds oil - and even if he uses oil in his possession that belongs to the others, he does not need to inform them. And certainly if he adds from his own food.
The food was entirely used up, and one must start from the beginning: if he uses their food, and it is a type of food different from the type they used originally - he must inform them, since it is possible that they specifically wanted a particular type. And if it is the same type of food - he does not need to inform them. And if he uses his own food and confers ownership upon them, even if it is a different type - he does not need to inform them.
"Nitosfu aleihem":
The mishnah says "nitosfu aleihem" - new people were added who become part of the alleyway. Here too he adds and confers ownership: he may add from their food, and he may take his own food and confer ownership upon them. But here he must inform these people that he has included them in the shituf mevo'os.
Explanation of the need to inform:
Some understand the end of the mishnah, which deals with the new people, as speaking of those whose entrance leads into one alleyway and also into another alleyway, so that they have a choice as to which alleyway to join. Since a person can only be a member of one alleyway at a time, therefore he must inform them - perhaps they have no interest in being part of this shituf mevo'os, and prefer to participate in the other shituf mevo'os.
According to this view, a courtyard whose entrance opens into only one alleyway and has no entrance to any other alleyway - there is no need to inform its residents, because it is to their benefit and a positive thing for them to be part of the shituf mevo'os, and they will certainly want to participate in it if they wish to carry.
In summary: in this mishnah we learned that if the food of the eruvei chatzeiros and shitufei mevo'os was diminished - he adds and confers ownership and does not need to inform, and in the Gemara the details of the law were clarified: whenever some of the food remained, he does not need to inform, even with a different type and even from their food; and when the food was entirely used up, he must inform only when he uses their food with a type different from the original. And if new people were added to the residents of the alleyway - he adds and confers ownership, but he must inform them, and according to one explanation this is specifically when their entrance faces two alleyways and it is in their power to choose, for if they have only one alleyway - it is to their benefit and there is no need to inform them.