Very few of the original enlistees were still in the regiment at Monmouth. Aside from deaths from disease and battle, all of the original enlistments from 1776 expired during the Valley Forge Encampment. Some of the original officers still remained, however, including Colonel Bowman. Some of the original recruits had also reenlisted. Monmouth, however, was the end of the road for the storied 8th Virginia, a unit that first began as a Virginia provincial regiment led by a pastor and still loyal (technically, at least) to the King. The Virginia legislature had intended it to be a German unit and commissioned German field officers for it (Col. Muhlenberg, Lt. Col. Bowman, and Maj. Helphinstine). It recruited men of other ethnicities, however, and was never as German as originally envisioned.
Some of the men, commissioned and enlisted, continued to fight on to the end of the war. The 8th was provisionally merged with the 4th and the 12th regiments during the summer of 1778 and fought at Monmouth with that designation. In September, the regiment was merged with the 4th Virginia under the latter's number. Other regiments were consolidated as well, and the regiment previously known as the 13th became the "new" 8th Virginia. In 1779, the consolidated 4th was provisionally merged with the 3rd Virginia and known for a time as the "3rd and 4th Virginia Regiment." Lastly, the handful who remained were included in the 2nd Virginia Brigade sent to reinforce General Benjamin Lincoln at Charleston, South Carolina, in 1780. Some of them were under the command of Captain Abraham Kirkpatrick, who had begun the war as a lieutenant in William Croghan's Pittsburgh company of the 8th Virginia. Croghan, now a major, was also at Charleston. All of them were taken prisoner when Lincoln surrendered on May 12, 1780.
Brigadier General Peter Muhlenberg, the regiment's original colonel, and Lt. Colonel William Darke, one of the regiment's original captains, were both at Yorktown. They may be the only men of the original 8th Virginia who served at Yorktown as members of the Continental Army. Darke, who had been captured at Germantown, paroled and then exchanged, held a Continental commission but led militia from Hampshire and Berkeley counties to Yorktown. Malaria, to which men from the frontier had no resistance, prompted his militiamen to return home before the surrender of Cornwallis, but he remained (presumably having developed a resistance after the regiment's ordeal in Georgia in 1776). Private Bean Smallwood, an original 8th Virginia recruit in Captain Berry's company, was at Yorktown as a militiaman.
Here is an excellent overview of the Battle of Monmouth.