Each owner of a DataGroup can link the group to another one. He or she can
then select whether the group should be a child or a parent of the other group.
In any case, an owner of the other group has to confirm the membership.
The figure Sequence Diagram to link two DataGroups in the parent graph illustrates this workflow for the case
that the owner of the child DataGroup requests the link. A
DataGroupRelation object is created (see also
DataGroup Relations) and the owners of the other
(parent) DataGroup get an email with a link to approve or reject the
request. If rejected, the DataGroupRelation object is deleted again,
otherwise the users of DataGroupUserGroups of the parent DataGroup
are added to the corresponding DataGroupUserGroups of the child.