Similar to soul mates who have shared a past life connection, a soul group is a group who have shared the same life experiences.
It will be a group that you immediately feel comfortable with. It may be found by accident, if you are drawn to a group of people.
You may also find your soul group by expanding your social networks or trying different things. You could also attend Spiritualist churches or attend psychic development lessons. You could also do a past life regression.
I feel that soul groups are much more difficult to find than soul mates. I have met some amazing people in my overseas travels and had some interesting conversations. I wonder whether they were part of my soul group.