The Group/formation(s) that a member is attached to will be defined by the information shared from your Branch Membership Systems through to Members Admin.
Scouts | Terrain is not replacing your current Branch Membership processes for moving section, Unit or Group! Make sure you follow the relevant Branch process for doing this. Once your Branch has moved you between formations, this will flow through to Scouts | Terrain as explained below.
Members can belong to multiple Units & Groups within Scouts | Terrain. For a full overview of how to do this, visit the member table how-to guide at https://pr.scouts.com.au/mdocs-posts/how-to-navigate-the-members-table/