def METHOD_NAME(self, matrix, group_id):
    """
    Get the individual rows belonging to a particular group without summing
    them
    Returns a matrix of shape:
        (number_of_rows_in_group X columns_in_original_matrix)
    """
    row_selector = self._group_selectors[group_id]
    group = matrix[row_selector]
    # See `is_matrix` docstring for more detail here
    if is_matrix(group):
        group = group.todense().A
    return group