The managers of a group may choose to create one or more subgroups in that group, as distinct from creating additional separate groups. With a few exceptions a subgroup of a group has the same features and controls as any other group, including the parent group.
It is those exceptions which matter when considering whether to make a subgroup to an existing group, or a separate group.
Related members and management
One reason to use subgroups is that it can ease the management of group memberships.
The Subgroups page shows all parent group members, with checkboxes to control their subgroup memberships (add or remove).
Subgroup members must be members of the parent group
People who attempt to join a subgroup, and are not yet members of the parent group, join the parent group instead. Then they may join the subgroup. If either or both the parent group or the subgroup have restricted membership then moderator approval will be required at that step.
There is no Invite feature in a subgroup, people must be invited to the parent group.
If a subscriber leaves or is
removed from the parent group, that subscriber is automatically removed
from all of the subgroups.
Direct Add
There is a Direct Add feature in subgroups. Unlike Direct Add in the parent group it is available even in Basic ("free") groups, but it applies only to members of the parent group.
Open or Restricted subgroup membership
Subgroup membership can be open to parent group members, even when parent group membership is restricted.
Related addressing
One reason to use subgroups is that their email and web addresses have a common identity with those of the parent group.
Subdomain addressing
Explain the new format. Cite Mark's beta@ announcement (soon) or most recent post.
Legacy addressing
Explain the deprecated +subgroup addressing scheme.
Access Controls
Features in the subgroup can be made visible to parent group members, while still being hidden from the public.
Examples
Some example use cases for subgroups.
Moderators' group
A common desire is to support better communication among the management of a group. In Y!Groups and other email lists this is sometimes achieved by creating a separate group, whose membership is restricted to just the moderators and owners of the primary group.
This is a natural use case for a subgroup.
PTA group
Primary group is all PTA members, subgroups for board, teachers, school admin, and students.
The official Groups.io user documentation is in the Groups.io Help Center.