See also the official documentation in:
Help Center > Owners Manual > Using subgroups

NOTE: Subgroups are generally available only to groups on the Premium or Enterprise plan.

If you don't keep current on your payment plan, your subgroups will be locked.

Groups on the Basic (free) plan that were created prior to January 15, 2020 are grandfathered, and are allowed to have subgroups.

See official help for note about subgroups for basic, premium and enterprise groups: https://groups.io/helpcenter/ownersmanual/1/customizing-settings-for-premium-and-enterprise-features/subgroups?single=true

If you are a large group, or have differing roles and responsibilities, or specialized interests, you may find that a subgroup will work well for your group.

A detailed page in this wiki in the Best Practices section - Why use a Subgroup - gives details about group addresses and more. This section is just an overview.

Creating a subgroup - the basics

You can create a subgroup for your main group if you have a Premium or Enterprise plan, as of January 15, 2020. (Basic groups existing on that date are grandfathered in. You can also create a new subgroup if you are "legacy enabled" / grandfathered)

For example:

1) Create avian group (this becomes the subdomain name once a subgroup has been created).

Now you have:

avian@groups.io - an ordinary group.

2) Within avian group, create birds subgroup.

Now you have:

avian@groups.io - The original group's original address continues to work (is aliased).

main@avian.groups.io - the original group.

birds@avian.groups.io - the subgroup.

(NOTE: There is a delay for access after you create the first subgroup, due to the creation of the new subdomain, in this case avian.groups.io.)

3) Go to the settings for main group and change its name (for example, to chickens). (There might also be a delay in being able to access this name because it will need to be propagated through the DNS system.)

Now you have:

avian@groups.io - the original group (alias).

chickens@avian.groups.io - the original group.

birds@avian.groups.io - the subgroup.

Email and web addresses

As you see above, one name ("avian") is shared by all groups, while the parent group and subgroup also each have a distinguishing name ("chickens" or "birds").

Membership

Members of subgroups must be members of the main group.

Members of main groups are not necessarily members of a subgroup.

As of Feb 2019, you can invite someone to join a subgroup (they will be added to the parent as well if they accept).

Members of the main (parent) group can be direct-added to a subgroup. This is true even in a Basic / Free group as long as they are already a member of the main group.

Subgroup Messages and Archives

Because subgroup members are also members of the parent group, they can send messages as members to either group.

If someone of the main group who is not a member of the subgroup sends a message to the subgroup, that message would be treated as having been sent by a nonmember (rejected or moderated, depending on your group's settings).

A subgroup's archives might be visible to the main group members, depending on the Privacy settings of the subgroup:

• Listed in parent group, publicly viewable messages
• Listed in parent group, messages viewable by parent group members
• Listed in parent group, messages viewable by subgroup members only
• Not listed in parent group, messages viewable by subgroup members only
• Not listed in parent group, publicly viewable messages

Member List

If you choose to have members see the members list, the main group members will see subgroup members because subgroup members must be members of the main group.

Existing Group as a Subgroup?

It is easy to create a subgroup. However, if you want to transform an existing Groups.io group into a subgroup, it might be possible, but you must contact support.

If you want to transform a Google Group into a Groups.io subgroup, create the subgroup and treat it as the destination of the transfer process, proceeding as you would for any other transfer.

Storage

At present, each group and each subgroup has the allotted amount of storage shown on the pricing page. The original plan was to have the limit apply to the total group, including subgroups. If you have a free group with 2 subgroups, you have a total of 3 GB available, but can't 'share' it amongst them (at least not directly).