After reading the previous original plus 6 replies, I feel we need to identify a few things first.
What is Groups.io? It is an entity that provides the ability to build online groups that may also function as mailing lists.
Ken, what you are asking for is the ability to have preset list to allow members of a group at groups.io to send emails to a subset of the members of a group (or subgroup?).
To me that is the current definition of a subgroup.
Or, much more simply, it is the definition of a mailing list i.e.a list of people's email addresses kept as a single unit on or available to one's own computer(s).
As J's postings and the lack of conversation/interest about the subject indicate to me:
Your idea/suggestion is quite incomplete, lacks clear & complete definition.
Why complicate groups.io's current operating mode with this "feature" when it's easily defined through functions in current email applications?