The Gemara in Masechet Eruvin (Daf 54) discusses the situation of a group of people standing in a box formation in a Rishut HaRabim (public domain) on Shabbat. Since the people formed an enclosed area, the Gemara says it would be permissible to carry within that area. Four "walls" made from human beings suffice to create a Rishut HaYachid (private domain), within which one may carry on Shabbat. In fact, the Gemara tells of a story where Rava had once left his Shiur (lecture), and an entourage accompanied him and formed an enclosed area within the public domain. His attendant took advantage of the situation by moving Rava's flasks of wine that had been left outside. Since the students surrounding Rava formed a Rishut HaYachid, the attendant was allowed to move the flasks of wine within that enclosed area.
Is this a viable method for carrying on Shabbat? If one wishes to carry items through a public domain on Shabbat, may he invite a group of people to stand in a box formation so that he can carry within the enclosed area?
The Shulchan Aruch (362:5) writes [listen to audio clip for precise citation] that one may indeed carry on Shabbat within an enclosed area formed by human beings, provided that they all stand within three Tefachim (handbreadths) of one another. He adds that even while the people move, if they maintain the box formation one may carry within the enclosed area. However, the Shulchan Aruch notes, Halacha does not allow one to utilize this method of carrying if the people forming the "walls" know that they are serving this function. Even if just one of the people is aware that the group was assembled for this purpose, would this method be nullified. If the people know that they serve as a "wall" to allow carrying on Shabbat, this will result in Zilzul Shabbat, an inappropriately relaxed attitude towards the laws of Shabbat. Therefore, one may not explicitly assemble a group of people to form a Rishut HaYachid to allow carrying on Shabbat.
Furthermore, the Shulchan Aruch rules that even if the group is not informed of the purpose they serve, but they will likely become aware of their function as a human "wall," one may not carry within the enclosed area that they form. Therefore, if a group of people was once used for this purpose, one may not assemble them again on a different occasion to form a Rishut HaYachid, as they will likely suspect the reason for their having been brought together.
Nevertheless, if one assembled a group of people to form a Rishut HaYachid, and as he carries within the domain they discover the reason for their having been assembled, he may continue carrying. Once he began carrying within the enclosed area before the people learned of the purpose they serve, he need not stop carrying when they become aware of their function.
The Rama (Rabbi Moshe Isserless, 1530-1572, Rabbi of Cracow; author of famous glosses to the Shulchan Aruch) imposes a significant limitation on this Halacha, commenting that one may utilize this method for carrying on Shabbat only in situations where one has a pressing need to carry. In the absence of a pressing need, one should not carry in a Rishut HaRabim by forming four "walls" from human beings.
Of course, it will rarely happen that a group of people will form an enclosed area within a public domain on Shabbat without being aware of the purpose of their formation. Nevertheless, one should be aware of this option for carrying on Shabbat in situations of dire need. Every so often a group of people accompanies the Rabbi out of the synagogue on Shabbat and indeed form an enclosed area, in which case one would be permitted to move items within that enclosed area, if he has a pressing need to carry. Of course, one must ask a Rav for the perimeters of the leniency before using this Heter.
