A group is an entity that is created for access control. A file can only have one “group” owner.
What is interesting about a group is its membership. A group can have any number of members. Each member of a group must be a user.
By controling how the “group” owner can access a file, and also the membership of the group owner, there is a great deal of flexibilities.
Note that a group member cannot be another group.
By default, most Linux distributions automatically create a matching group when a user is created. For example, when user newbie is created, a group with the same name is created.
If the user owner of a file is also a member of the group owner, then the user owner permissions take precedence.