I've pushed subgroups to the live site. To create a subgroup, click on the '+ Subgroup' tab when viewing your group. Here's the text I've written up for the help center:
Groups.io supports subgroups. A subgroup is a group within another group. When viewing your group on the website, you can create a subgroup by clicking the 'Subgroup' tab on the left side. The email address of a subgroup is of the form email@example.com.
Subgroups have all the functionality of normal groups, with the exceptions that to be a member of a subgroup, you must be a member of the parent group, and you cannot invite people to join subgroups.
Once a person is a member of the parent group, they can subscribe directly to the subgroup if it's permitted, they can be added directly to the subgroup, or they can ask to join the subgroup if it is restricted.
Please let me know if you see any problems or have any suggestions for improvements. I'll probably announce this on firstname.lastname@example.org Monday or Tuesday.