I second the kanban-like approach (have an affinity for trello myself).
Another thing I’ve found really helps is if you can link to something to reverse engineer, even if it’s just an aspect of the overall app. That way everyone is looking and thinking about the same thing and it becomes much more accessible for people to contribute. Also using numbers for features helps make it more structured as well.
So at the end everyone has had an opportunity to contribute and you’ve got a features list which can then be translated into tasks on trello, and then team members can pair off and put their names on each task (if that’s how the team is structured).
TL;DR: Finding either another app (or at least something concrete everyone can imagine and work from) to reverse engineer and go from there really helps in group settings, especially for beginners. @codejunky showed me this style and it has worked pretty well in practice.
Not sure if that helps, but I hope so!