The news comes a day after Hicks was interviewed for nine hours by the panel investigating Russia interference in the 2016 election and contact between Trump's campaign and Russia.
He only briefly mentions the accusations, but says he is resigning to "focus on what is most important in my life; my marriage, my family and my health."
One of those people said Spicer is quitting because of objections over the appointment of a new White House communications director, New York financier Anthony Scaramucci.
Kroeker is the owner of Casper Mountain Motorsports and he said in an email to his colleagues that his business will not allow him to take time off for this year's session.