In reading "group polarization", "
that a person tends to shift to a more extreme opinion when in a group setting than what his/her original opinion might have been"
So, in the case that people change their opinion and follow the main dominated opinion of the group is called "Normative Influence" or "Informational Influence". However, I did not see the name of the phenomenon that the person state their opinion more firmly after the group discussion. Example from the link above
To state an example, let’s say that a person thinks that child beauty pageants―contest featuring under-age children―are not a very good idea. This opinion that he/she harbors could be a very mild one, where she might not really have given it a lot of thought. However, if she’s placed in a group and this topic is taken up for discussion, there is a very high possibility that by the end of the discussion, she will not only be completely against child beauty pageants, but she may begin stating this opinion more firmly, and maybe, even join a protest group.