I will lay out my thoughts on groups, their pros and cons, and some examples. My libertarian leanings are going to be quite evident. If you're on your own, then you're not part of a group and that is an entirely different situation.
TeamA team is two to five people who are working toward a common goal. Trust is paramount to a team; they are not very effective at reaching goals if there is no trust between the members. Teams are limited in the scope of what they can accomplish by the fact that they are rarely self-sustaining. Teams are the lowest level of group that offers security to its members. Examples include a work group, search team, or small family.
TribeA tribe is a collection of people (or teams), usually numbering between ten and twenty-five, who have something in common and who work together to support each other. A tribe can be an extended family, a collection of small families, or other group of like-minded people who are willing to work, and probably live, together. A sufficiently large tribe can be self sustaining, providing for the needs of the tribe by itself. History is full of different examples of tribal structures; the length of time that they existed is a good gauge of how effective they were at dealing with the problems inherent in groups of people.
Some writers place an emphasis on race or religion when describing a tribe. I don't concur with the racial separatists. There is only one race of humans: homo sapiens. The phenotypes that separate us into Caucasoid, Negroid, and Mongoloid are often used to divide us as are religious difference, sexual orientation, language, and ancestry. "Divide and conquer" is an old tactic that you have to watch for, because if someone is trying to split you apart from others, it is often so they can have more control over you.
"There are men in all ages who mean to govern well, but they mean to govern. They promise to be good masters, but they mean to be masters." -- Daniel Webster
Town or TownshipA town is a collection of families and/or tribes in a small area that have gathered to facilitate trade and/or manufacturing. Most towns are formed around routes of transportation, resources, or water. Towns are easier to secure than a township, but the concentration of resources and people make them more susceptible to damage from disaster and more likely to be seen as a target in times of conflict.
A township is more rural - a collection of families and/or tribes in a disperse area that trade and communicate with each other. Usually agriculturally based, townships provide less security than a town but offset this with greater freedom and resilience. A tornado or fire can wipe out an entire town, but a township will likely have some surviving resources to help with rebuilding.