According to the letter of the law, whenever people get together to eat or drink in a kavua fashion, one person should be motzi the others even if they all know how to bless. This would include a bread meal, fruit brought out during a meal, and places where people gather just to drink wine together. The idea is that one blessing for all gives more glory to the king. (Rama1 – MB3,11)
Nowadays, in most places the minhag is that no one is motzi others on anything. This may be because not everyone has the ability to have kavana to be yotzay and to be motzi others. (MB12)
There are two cases that we go according to the letter of the law:
If the listener does not know how to bless. L’chatchila they should be sitting together. B'dieved, as long as the blesser had in mind to be motzi and the listener had in mind to be yotzay, it’s valid.
Beracha achrona especially May’ayn Shalosh. It gets disgraced by many people – so one person should be motzi everyone – even those who know it. It is best for those who know the beracha achrona to say it along word by word. (MB9 – ShaH7)
After hearing a beracha from someone being motzi you, there are two reasons to answer omayn: one is to perform the mitzva of answering omayn after any Jew’s beracha. Another is to actively affirm the beracha that you had kavana to fulfil. If you didn’t answer omayn, you still fulfilled your obligation with shomaya k’oneh. (ShA2 – MB17)
If you didn’t hear a word, it is like you didn’t say that word. So, if the word is necessary for the beracha (e.g. Boruch) you were not yotzay. But if it isn’t from the ikkar roots of the beracha you can be yotzay without it. (MB19 – ShaH20)