This word in its statistical sense should be used with discretion. There is a coldness and inhumanity in describing people as groups, though it is not so bad as describing them as brackets. The writer of the sentence
"these are likely in the main to be bought by the lower income groups"
would have expressed his meaning more plainly if he had written
"these are likely in the main to be bought by people with small incomes".
Income Group has become a cliché.