A “group” is a construct that is intended only for access control. There is no way to authenticate against a group. For all practical purposes, a group should have at least one user as a member. However, it is possible to set up a group with no members.
Members of a group are limited to users. This means it is not possible to add a group as a member of another group. However, a user can belong to any number of groups. As a result, altough groups are not hierarchical, a system still has the flexibility to assign any user to any group.
By default, the adduser command adds a user, and automatically create a group of the same name.