When people are in groups there are two things that are happening.
One is that people do as a majority of the group is doing. They do not express their own opinion which may differ from that of the group as they are afraid of the opposition from the group. This happens even if a large number of people in the group have a differing opinion, unless one of them stands up as a leader to represent them.
The other thing is that people do things which they would not do if they were alone as they feel they are protected as a part of the group from being identified individually. This leads to the mob mentality as the earlier person mentioned.
People do things in groups that they would not do on their own. This can be both good and bad. Groups stretch people beyond their limitations. By working together, people can achieve more than they can on their own. On the other hand, sometimes the mob mentality kicks in. People often do things that they would never do alone, just because others are doing it.
That is a great question and I cannot give an adequate answer in such a small space. But here goes. First, you need to remember that each group has a commonsense view of the world. In other words, society is socially constructed. Second, no two groups share the same commonsense view of the world in perfect harmony. There can be overlap, but there could be great differences as well. When these societies comes together the more powerful group's point of view or worldview will be dominant.
Now for an example, if you live in a modern enlightened, city in the North East (America), most people will have a relativistic view of the world. However, if you were a person from a foreign country, who group up in a very dogmatic society, then you would not fit in all that well. The larger groups opinions will push upon you. By examining this example, you will be able to think of plenty of other examples.