Social psychology is very vast. Just like political parties. For example the Democrats in USA, the strong influence the group still have on individual.
An Individual on a group. The king of Saudi Arabia has influence on the country's affair and as much people look up to him for leadership.

Group on Group, like a rally or protest. One group picks inspirations from another group and was deeply influence by their activities.

Humans in general have influence on each other. We all what motivation speaker like Luther King did in the US. He influence them. That why man is known as a Social animal