Members come together to cooperate for a common purpose.
The common purpose is more important than self-interest.
They act as one person. From the point of view of outsiders, all people in one UDG are the same, interchangeable.
This group has a perpetual life. As people die, they are replaced by new members who are the children of members.
As Henry Maine said, 'corporations never die'. UDGs are corporations – building blocks of a society in which people's relationships are based on status.
Interactions between people of different groups are interactions of different groups. This is not exactly strangerhood, but it is very different than the relationships within the group. This is the public domain, as opposed to the domestic.