Usually the smaller group, the better. I like it with either one other person or sometimes two. Any more than three or four is extremely difficult to follow what's happening and what everyone is saying. This includes the background noise of someone else's conversation, or even the radio.
When I'm only talking with one other person, the conversations tend to be more meaningful and more personal. When I'm talking with two or three people, it's still pretty relative and important, but with four or more, it's time to call in the tangent police.
Tangent Police, by definition, are people who are appointed within a group conversation to keep the discussion on-track and pertinent. They have the authority to point out when the discussion has become unimportant and punish violators after a few warnings, depending on the situation.
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Itaque incipet.
All that glitters is not gold but at least it contains free electrons.