A small group could be defined as a small number of people (usually 4 - 12 people) that meets deliberately and regularly for relationship building and some other purpose. There is a deliberately human element to a small group.
Small groups in the modern era are most commonly used in faith and self-help communities, with huge success and in huge numbers: hundreds of millions of people worldwide would count themselves as part of one. The model is used for a huge variety of community building, group counselling, and high-performance delivery purposes. They are well established as a highly effective model for social change. Most social change movements are, arguably, born out of small groups. As Margaret Mead, the famous anthropologist put it:
“Never doubt that a small group of thoughtful, committed citizens can change the world; indeed, it's the only thing that ever has.”
The beauty of small groups is their flexibility and adaptability for both the people who attend them and the contexts they exist within. No two groups are the same but they have sufficiently similar characteristics that allow them to multiply whilst maintaining community.
What small groups are not
To understand what small groups are, it may also help to mention what they aren’t:
Small groups are not single person led educational environments. They are certainly used to learn things, but they are not dominated by a teacher. They may well be facilitated and hosted, but their value lies in collective learning and exploration.
or small groups to be successful they look to evolve; in their application, content, depth of relationship and, potentially, membership and size. If they stay static for too long they get into a rut, start to die and reduce their effectiveness.
If a small group goes beyond 10 people (and at most 12), it starts to lose its power because there are too many relational networks to manage. If a group has grown to this size then that is a sign of success, but it is also the point at which the group should ideally split into two or more groups.