The terminology was made popular by Henri Tajfel and colleagues during his work in formulating social identity theory. The significance of ingroup and outgroup categorization was identified using a method called the minimal group paradigm. Tajfel and colleagues found that people can form self-preferencing ingroups within a matter of minutes and that such groups can form even on the basis of seemingly trivial characteristics, such as preferences for certain paintings.

Who introduced the term in group and out group? Let's have some stream of consciousness first.
The relevant sentence in the passage is: The terminology was made popular by Henri Tajfel and colleagues during his work in formulating social identity theory. So, the answer is Henri Tajfel and colleagues.