This (gasp!) highly subjective node is based on my own experience; your mileage may vary
Picture 4 people sitting at a table (A, B, C & D). Now imagine a general discussion on many different topics. In an optimum conversation, the amount of "vocal time" (the amount each person speaks) will vary with time, but will average out to be 25% of the total:
Division of conversation time:
A: 25%
B: 25%
C: 25%
D: 25%
However, the two models of conversation time I have most frequently come across is as follows:
The withdrawn participant:
A: 35%
B: 30%
C: 30%
D: 5%
The 2-way with observers:
A: 40%
B: 40%
C: 10%
D: 10%
In the withdrawn participant D is detached from the conversation flow, for whatever reason (Depression, eating, lack of knowledge of subject, etc). In the 2-way with observers, A and B will conduct most of the conversation with C and D interjecting comments.
I have noticed another model of conversation, when a fifth person, E, joins. E immediately attempts to monopolise the discussion. The reasons for this are many: maybe E is a manager or Alpha Geek, maybe E is insecure and needs to dominate to feel happy, or maybe E is over enthusiastic and has misjudged the current conversation model:
The Dominant newcomer:
A: 10%
B: 10%
C: 5%
D: 5%
E: 70%
While the optimum with 5 people would be 20% of "vocal time" each, the dominant newcomer essentially ruins the threads and flow of the current conversation, turning it to their own purposes. If the DN has a particular subject they want to discuss, they will simply change the subject (How they do this depends on their grasp of subtlety). If the DN is simply over enthusiastic, they will excitedly interject into a thread, then monopolise that conversation without realising.
The dominant newcomer may annoy the other people present to the point where they will actively avoid the DN or attempt to shut the DN out of the conversation. If a more socially mature person is present, they will deflect the DN long enough for other participants to get a word in edgeways.
Keep this in mind the next time you go to the dinner table, the coffee house or the pub. You may find yourself as the dominant newcomer, annoying others.