Question: The Immortal Ten were a group of free-staters that included a Kansas Territory Jayhawker who joined what during the American Civil War?

Context paragraphs:
Washington in the American Civil War: The history of Washington in the American Civil War is atypical, as the territory was the most remote from the battlefields of the American Civil War.  The territory raised a small number of volunteers for the Union Army, who did not fight against the Confederate States Army but instead maintained defensive positions against possible foreign naval or land attacks.  Although the Indian Wars in Washington were recent, there were no Indian hostilities within the area of modern Washington, unlike the rest of the western states and territories, during the Civil War.  At the start of the American Civil War, modern-day Washington was part of the Washington Territory.  On March 3, 1863, the Idaho Territory was formed from that territory, consisting of the entirety of modern-day Idaho, Montana, and all but southwest Wyoming leaving the modern-day Washington as Washington Territory.

John Wilkins Whitfield: John Wilkins Whitfield (March 11, 1818 – October 27, 1879) was a territorial delegate to the United States Congress representing the Kansas Territory from 1854 until 1856.  He was an officer in the Confederate Army during the American Civil War, being commissioned as a brigadier general on May 9, 1863.

John W. Geary: John White Geary (December 30, 1819February 8, 1873) was an American lawyer, politician, Freemason, and a Union general in the American Civil War.  He was the final alcalde and first mayor of San Francisco, a governor of the Kansas Territory, and the 16th governor of Pennsylvania.

The Immortal Ten: The Immortal Ten were a group of militant Kansan abolitionists and Free-Staters who on July 23, 1859, freed John Doy from a Missouri jail, where he was being held for allegedly abducting slaves.  The group included George R. Hay, Silas Soule, Charles Doy, Capt. John E. Stuart [Stewart], S. J. Willis, Thomas Simmons, Joseph Gardner, Jacob Senix, Capt. Joshua A. Pike and Major James B. Abbott.

Silas Soule: Silas Stillman Soule (July 26, 1838 – April 23, 1865) was an American abolitionist, Kansas Territory Jayhawker, anti-slavery militant, and a friend of John Brown and Walt Whitman.  Later, during the American Civil War, he joined the Colorado volunteers, rising to the rank of Captain in the Union Army.

Alexander Bullock: Alexander Hamilton Bullock (March 2, 1816 – January 17, 1882) was an American lawyer, politician, and businessman from Massachusetts.  First a Whig and then a Republican, he served three terms (1866–69) as the 26th Governor of Massachusetts.  He was actively opposed to the expansion of slavery before the American Civil War, playing a major role in the New England Emigrant Aid Society, founded in 1855 to settle the Kansas Territory with abolitionists.  He was for many years involved in the insurance industry in Worcester, where he also served one term as mayor.

Marais des Cygnes massacre: The Marais des Cygnes massacre ( , , or ) is considered the last significant act of violence in Bleeding Kansas prior to the outbreak of the American Civil War.  On May 19, 1858, approximately 30 men led by Charles Hamilton, a Georgian native and proslavery leader, crossed into the Kansas Territory from Missouri.  They arrived at Trading Post, Kansas, in the morning and then headed back to Missouri.  Along the way they captured 11 Free-Staters, none of whom were armed and, it is said, none of whom had participated in the ongoing violence.  Most of the men knew Hamilton and apparently did not realize he meant them harm.  These prisoners were led into a defile, where Hamilton ordered the men to shoot.  He even shot and fired the first bullet himself.  Five men were killed.

James Montgomery (colonel): James Montgomery (December 22, 1814 – December 6, 1871) was a Jayhawker during the Bleeding Kansas Affair and a controversial Union colonel during the American Civil War.  Montgomery was a staunch abolitionist and used extreme measures against pro-slavery populations.

Jayhawker: Jayhawkers is a term that came to prominence just before the American Civil War in Bleeding Kansas, where it was adopted by militant bands affiliated with the free-state cause.  These bands, known as "Jayhawkers", were guerrilla fighters who often clashed with pro-slavery groups from Missouri known at the time as "Border Ruffians".  After the Civil War, the word "Jayhawker" became synonymous with the people of Kansas.  Today a modified version of the term, Jayhawk, is used as a nickname for a native-born Kansan, but more typically for a student, fan, or alumnus of the University of Kansas.

Rockville's post: Rockville's post, in southern Miami County, Kansas, was established at the small town of Rockville, Kansas, founded in 1859 by those loyal to the southern cause in Kansas.  The other side, the free-staters, soon gained control of the town and it was loyal to the Union when the Civil War broke out in 1861.  Rockville was located on the top of a hill surrounded by rolling plains.  During the American Civil War, the area was almost bare of trees, allowing troops holding the town to see anyone coming from some distance away.  Rockville's post was one of the many posts established in the War to help guard the Kansas-Missouri border area.

Supporting paragraphs: The Immortal Ten Silas Soule
Answer: Colorado volunteers