Question
Is one required to have the leader of a zimun bench over a cup of wine? I have seen some people do this but most homes I have been at do not do so.
Answer
The Shulchan Aruch (או"ח קפב:א) brings 3 opinions.
1.That even an individual must bench over a cup of wine. On this particular opinion the Mishna brura says that today's custom is that no individual should bench on a cup of wine.
2. Only when 3 eat together it requires a cup of wine.
3. Even 3 people are not required.
The Shulchan Aruch himself did not determine what should be done. The Rema says that it is a "mitzvah min hamuvchar" –meaning it is not an obligation but is a choice mitzvah to bench over a cup of wine.
The Mishna brura says that today's custom is to adopt the third opinion that there is no obligation and this is very common.
Nonetheless at a "se'udat mitzvah" when there are 10 or more people, many people bench over wine. Therefore we see different customs and everyone has a justified source.