Although you cannot create sub-groups, you can create as many groups as you wish. The same people can be in multiple groups. 

You can create the main group with all members, then create new groups to serve as sub-groups, and invite your members to join whichever sub-group you want them to join.

For example, you might have a main group with 24 members. Then you can create 3 small groups with 8 members in each one. Yes, this means that each of your clients will be a member in 2 groups.