Robert,
Ah I don't know about sub-groups! Is this something that has to be
created within a group you create on Groups.io,
Yes, a group may have one or more subgroups.
Group owners can use the +Subgroup page (in the list at the left) to create new subgroups. Mods or members may be able to also, depending on the "Create Sub Groups" option on the group's Settings page.
and as a special request?
Yes, at Step 2 in the Easy Group Transfer process, when you send the "go ahead" email to transfer @ groups.io be sure to name the subgroup as the destination for the messages, and let them know that you do not want the members transferred.
-- Shal
Shal Farley