The import process will create all non-existing groups. If the user already exists and the "add_existing_users_to_groups" switch is NOT set, then the line is skipped (so if you specify a nonexistent group for this user, the group won't be created). If you enable "add_existing_users_to_groups" and the user is already present, the nonexisting groups are created and the user is assigned to all the specified groups.