It is a known (spiritual) fact that souls incarnate in groups. Groups of souls can be of many kinds and exist for a variety of purposes. But these purposes all have to do with karma, the universal law of cause and effect.
As I've previously outlined, karmic law is non-linear and is not a merit-punishment law. Rather, it is multi-directional (even multidimensional) and is a cause-effect law. As a soul in incarnation, a person can be part of many soul groups at the same time. Our immediate family can be a group. The friends we grew up with can be a particular group. A community (religious/spiritual, professional, etc.) can be a particular group. Even a whole ethnic or national group can be that. Those are possibilities. They don't necessarily qualify as soul-groups each and every time. Why? Because soul groups are united by soul purpose, which has to do with specific karmic necessities.
Let me explain while keeping in mind that any explanation concerning soul reality is inexact to some extent, for soul reality exists beyond the limited scope of anything conceptual.
Soul purpose always has to do with something like growth, or evolution, or expansion. In other words, soul (or consciousness) seeks to ever expand the scope of its freedom, of its capacities, of the possibilities for further experience and expansion. This expansion is a very long and most difficult process because this world of physical and psychical matter acts both as a means and a hindrance to this expansion. Such hindrances are the attachments we form with other people, towards other people, towards various endeavors involving other people.
Take for example a particular group of souls whose expansion into matter is hindered by an abnormally strong sense of attachment towards their own ethnic, national, or cultural group. In other words, their incarnated consciousness is strongly identified with something that has to do with a sense of “us-pride”, based on skin color, or religious belonging, or cultural history, etc. The main idea here is that consciousness is ignorant of its real self, of its own nature, and believes stuff like “I am that appearance”, “I belong to such and such group”. From incarnation to incarnation, the soul remains incapable of realizing that this sense of belonging to a temporary group is not all that significant. Sure, it is totally okay to love and be proud of one's culture, nation, and such. But, here we're talking about something abnormal, meaning that this type of love and pride has devolved into some version of “us against them” (others). There are, obviously, various degrees of that kind of attitude. Yet it is all based on superficial and temporary characteristics.
Now, when many reincarnating souls spend lifetimes cultivating a form of disdain, fear, prejudice, even loathing towards any group that feels different in any way from their own, they tend to group together and gradually reinforce this kind of negative attitude, until they learn to let go of this kind of attachment. Pockets of intense political rivalry, constant problems between racial or ethnic groups, or wars, massacres, genocides, are all the karmic result of such kinds of attachments. Consciousnesses who allow themselves to harbor any abnormal sense of us-pride devolving into a sense of antagonism towards others may not wish to actually harm anyone belonging to the other group… for now. But, carrying this within themselves will cause them to naturally (that is karmically) be grouped or link with others who do the same, in this incarnation or the next. And next time, this sense of us-pride may be stronger, for it was previously (and unconsciously) cultivated. Any path will lead... further down that same path. The thing is, those who are similar psychically or who have the same attitude can (and many will) be part of the group that is viewed as different or antagonistic. Souls belonging to a particular group (here, being a group of souls attached to an abnormal sense of us-pride), are never really that far from each other, and so, sooner or later, they must attract and face each other, mirroring each other's antagonism.
We must remember that karma, the law of cause and effect, governs psychical phenomena as well as physical conditions. These two are ever linked, and causes emerging from the psychical nature eventually result in physically observable conditions, relationships, events, etc. In our endeavor to free ourselves from anything that hinders the clarity, expansion, and further enlightenment of our consciousness, we must not harbor undue attachments towards superficial characteristics such as race, nation, cultural group, or anything analogous. We must not forget that such attachments will necessarily come with their karmic consequences.