There is a famous article in the annals of psychology that found most people are typically capable of remembering seven numbers at any given time. That's why we have seven-digit phone numbers and seven-digit social security numbers.
Seven (plus or minus two), it turns out, is also a magic number when it comes to creating productive group dynamics.
Consider what happens when you invite too many, or too few, people to a meeting. Let's use our magic number seven (plus or minus two) as a guide.
If you have more than nine people (7 plus 2) at a meeting, perhaps 12, for example, it becomes difficult to give everyone a chance to be heard and to contribute. It's also easier for people to hide and not share their views. You'll also notice that whenever a big meeting like that slows down for a little while, people start side conversations as a way to be heard. Simply put, by inviting too many people, you didn't fully engage the group mind.
Add in the fact that it becomes exponentially harder to find a meeting date and time to get more people together, and it's easy to see why inviting too many people to a meeting quickly becomes counterproductive.
Conversely, if you invite less than five people (7 minus 2) to a meeting, you'll find that it's difficult to generate any energy within the group. It becomes too easy for a single individual or two to dominate the conversation rather than getting everyone fully engaged in contributing.
When you have between five and nine people working together, however, everyone can engage, no one can hide, and everyone leaves at the end feeling like they just participated in a productive exercise.
This rule is particularly applicable to when you might be trying to tackle high value work like strategic planning, which is a task that benefits from having smaller, more engaged groups working together rather than a large number of contributors.
You can also use the same rule the next time you're putting together an invite list for a dinner party. If you stick with the seven (plus or minus two) rule, you can create focused conversations around the dinner table that everyone can participate in. Invite ten or more people, however, and you'll find that multiple conversations begin as the group begins to break into smaller dynamics. Of course, that can be just fine--you just need to realize what you're going to get.
Just remember that when it comes to creating fully engaged and focused groups working on an issue, the magic number is always seven (plus or minus two).