Answer: “The heart of the group” are common desires that are able to exist in a mutual and devoted connection to one another. Together they can use the egoism that each person has risen above and bring these desires into sacrifice. That is, they now have work in the Temple, in their common house, their common desire. It’s within this adhesion to one another they discover adhesion to the Creator.
The unity between friends is a vessel and as a result of all these connected desires the adhesion to the Creator is revealed. It is said: “Through Your actions we shall know You.” The Light is revealed within the vessel.
From the 3rd part of the Daily Kabbalah Lesson 9/12/12, The Study of the Ten Sefirot