John Brown was asked by Brigham Young to oversee the migration of those groups that became known as the Mississippi Company of 1846 from Mississippi to the west. The intent was to meet the main body of members of The Church of Jesus Christ of Latter-day Saints along the Platte River. John traveled west with these saints in 1846, while leaving his family, as directed, in Mississippi. When the Mississippi Saints settled in Pueblo for the winter, John returned east in September 1846 to make arrangements to bring his family west. In January 1847, he was asked again to leave his family behind so he could go to Winter Quarters with other men to help protect the twelve and their families as they migrated west in 1847. John Brown was obedient to this request and arrived in the Salt Lake Valley on July 22, 1947 with the advance group of Saints under the direction of Brigham Young. John then returned east and finally brought his family west in 1848 to join the saints in the Salt Lake Valley.
There are many sources to find out more about John Brown, including the FamilySearch website:
https://www.familysearch.org/en/tree/person/about/KWJC-KNF