It is this sense of mutual interaction ...

[GoldfishLord]

In everyday life, we tend to see any collection of people as a group. However, social psychologists use this term more precisely. In particular, they define a group as two or more people who interact with, and exert mutual influences on, each other (Aronson, Wilson and Akert, 2002). It is this sense of mutual interaction or inter-dependence for a common purpose which distinguishes the members of a group from a mere aggregation of individuals.

Source: Sport and Exercise Psychology: A Critical Introduction — Aidan Moran

I'm still having trouble understading those parts.
1. Does "It" refer to the group or is a "dummy it"?
2. Does "sense" mean "meaning"?

I'd appreciate it if you would help me.

 

[emsr2d2]

I'm still having trouble understanding those the parts in bold.

1. Does "It" refer to the group or is a dummy "it"?
2. Does "sense" mean "meaning"?

I'd appreciate it if you would help me. Unnecessary.

1. It's a dummy "it".
2. It's more like "idea".

 

[Holmes]

It is this sense of mutual interaction or inter-dependence for a common purpose which distinguishes the members of a group from a mere aggregation of individuals.
. . .
1. Does "It" refer to the group or is a "dummy it"?

This is an "it"-cleft sentence, one in which the subject of the underlying sentence has been focused. I'm pruning off some prepositional phrases:

Underlying Sentence: This sense distinguishes the members from a mere aggregation.
"It"-Cleft Transformation (Subject Focused): It is this sense which distinguishes the members from a mere aggregation.

Another type of cleft sentence (the "pseudo-cleft" construction) yields this when the subject of the same underlying sentence is focused:

Pseudo-Cleft Transformation (Subject Focused): What distinguishes the members from a mere aggregation is this sense.

Both types of cleft sentence can be used to focus other elements in the underlying sentence besides the subject. These examples use the "It"-Cleft:

Direct Object Focused: It is the members that this sense distinguishes from a mere aggregation.
Prepositional Phrase Focused: It is from a mere aggregation that this sense distinguishes the members.