Polygamous "marriages" were conducted in secret by Joseph Smith as early as 1833/35 (even though the sealing power wasn't restored until 1836 and D&C 132 outlining the rules of polygamy was not given until 1843) and by a few select members beginning in 1843.
These men were also known as "Prisoners of Conscience." Why were these people, including First Presidency counselor George Q. It was even illegal in Canada and Mexico as they only recognize marriages that are legal in the person's home country.
Here is an overview of the dearth of Church-produced literature about Joseph Smith and polygamy before the Church published their essays starting in 2013.
This page was originally written before the Church released its three essays concerning polygamy. Some LDS adopt other views Why didn't angel visit Emma?
Polygamy as a doctrine was introduced into the LDS Church by Joseph Smith.
The practice continued after Joseph’s death in 1844, and was publicly announced in 1852, five years after the Saints arrived in Utah.