Missing a getUsernamesFromGroup() function in Fn collection

My difficulty
When designing a workflow in Nuxeo Studio, I cannot use the multiple tasks node and easily assign a task to every member of a specific group.

If I assign a task to group:groupName, then one task is created for the whole group. If I use "compute additional assignees", then I don't have access to something that will get me all members of a group easily.

What I would like to have
A function in the Fn object named getUsernamesFromGroup (or similar) which would create a String list with all members of a specific group, so I can use that in the "compute additional assignees" field.

Ideally, I would like this function to include a boolean parameter in order to choose if I want to ignore sub groups or not.