Individual competitions are between individuals.
Team competitions are between groups (or in the case of Program to Program competitions, between entirely different companies/locations/etc). In order to start a team competition, you have to be the administrator of a team.
If you're running a team competition on averages, people who do not enter a step entry weigh against the team average -- thus it's vital that you have high participation in your team.
Another point worth mentioning is that with all competitions, data in competitions is only tabulated between the competition dates. However team/group data is tabulated in an ongoing manner, no matter if that team is in a competition or not.