Hi,
note: I'm not the developer.
You don't assign users to a to-do list, but you do assign users to individual tasks. A workaround for you is to create a to-do list for the overall task, and break out the elements of that task to individuals.
Another workaround is to create a single user for your subgroup and assign tasks to that single user. In the email field for that user write all their email addresses with comma in between. mymail@example.com, secondmail@example.com
Typically tasks are achieved by individuals in project management software. I'm interested in examples of tasks that would have multiple assignees.