I have been part of attempts to build many teams throughout my career. In fact, I really enjoy building them, bringing out the best in people, and building a greater organism that brings with it friendships and learning. Some teams have come together and through thick and thin achieved amazing feats, while others behaved worse than a random list of names on a piece of paper. Some acted like elite special forces units, some like an eccentric family, while others like two year olds on the first day of nursery school or worse strangers on the Tube. While building a team isn't necessarily something that can be done in a prescriptive way, it in time be a lot like cooking. In fact, like Iron Chef, it is almost like being given various, sometimes almost random, ingredients to combine in various ways to attempt to create a delicious feast.
The best teams were rarely made up of the best from a talent and skills perspective, but rather those who were willing to put in the heart and effort, against the odds, to help the group. They came together naturally rather than being artificially put together based on some formula. There were always missing skills, things to learn, and stuff that looked set for failure that sometimes did. Roles sometimes seemed fuzzy, and would sometimes change frequently to keep the team balanced and on track, as well as sometimes providing the added benefit of ensuring that experiences (and pain) were shared. Nicknames often were given to people, places, things and ideas (I have had many bestowed onto me). Bread was broken, drinks shared, ideas constantly flowed, and it was quite normal for work and personal life to intermingle at times. I have had many key strategies and architectures drawn on a series of napkins and the back of random junk mail at people's homes and at random family style restaurants. Disagreements would abound. In fact, impassioned arguments always seem to be one of the biggest signs of success. Working through the heat of passion of people who really cared and who were allowed to confront each other in the desire to find the best way through. Ideas, discussion, and feedback must be constant, like blood flowing through the body. If feedback doesn't flow, or worse puddles in a silo, the team dies.
Teams have been of a wide variety of sizes. I have had organizations of many hundreds, and sometimes consisted of several teams, but I would not call any of the organizations themselves a "team". Organizations can pull together through alignment to a common vision, series of working groups and aligned teams, and occasionally "nested teams" where there might be some folks who have a natural knack for being able to be members of or straddle more than one team that has related goals and can cross pollinate both to create something that is far more than the sum of the parts. From experience the team itself is usually between 6 and 12 people, preferably 8 (+/-2), which seems to follow a lot of the common anthropological data on effective human groups out there. A team with any more than 12 never seems to gel, as it allows for people to overly specialize, or hide/be overshadowed by others.
Above all, the most important item is having a common purpose or goal that everyone shares and believes in. The shared thread, whether it be a project, a product, or a transformational mission, must be shared and believed in by everyone. That doesn't mean that people can't have different interpretations and opinions, even strong and contradicting ones. They also do not necessarily have to agree on the particular path. However, the belief in the high level end goal needs to be the same, and must be nearly a mantra. If it is not, like the two year olds the team will never gel.