It's like the atmosphere that envelops our planet earth. It is one. There is a famous pot example in Vedas. Imagine an empty earthen pot. There is air inside the pot. Now the same atmospheric air appears to be trapped inside the pot, however if this pot breaks then the air inside pot still remains part of the atmosphere.
Similarly all living beings are like earthen pot and one with Supreme Consciousness. When one dies this Supreme Consciousness remains and the body goes back to air, water and earth from where it came.
When someone dies in our neighborhood there is a grief stricken family. At the same time there is also a new baby born in the neighborhood where a family is rejoicing.
So if we have been joined in this group as one, there is a clear possibility that sometime, maybe hundreds of thousands of years ago we all lived in a one neighborhood.
I am you, You are me, We are ONE.