Good question. In this war, the Crown forces or the British army was made up of two distinct establishments: The regular British and the American Provincials. For the most part the British raised and recruited Regular forces in the British isles, while the Provincial recruited in the colonies. The colonies included everywhere from the 13 we think of, Canada, and Florida. Loyalist men had some options on what type of units they wanted to join including militias and provincial regiments, and at times Regular regiments.

Forming a militia or other form of corp took much more resources than most of the Loyalists could provide. Some of the commanders had to pay out of pocket for equipment and payroll until the British sent funds/equipment. The militias also stuck around counties as their area of operations and rarely ventured farther than that. Most Loyalists would only have to fight a certain amount of time which often was around a year. I don't have statistics on how many did a year or more. Militias did not have a standard uniform at the time and the British rarely provided any for them in the colonies. At most the British equipped them with a "stand of arms" including musket, bayonet, and cartridge box along with powder and lead. Loyalists definitely could pursuit this option if they solely wanted a shorter term of service. I'm only familiar with more southern militias but they did seem to serve through out the war in each state.

I would argue the Loyalists mostly enlisted into Provincial forces by war as a means of service to the King. These included regiments such as the British Legion, the New Jersey Volunteers, Queen's Rangers, King's American, etc. These regiments recruited heavily in the northern colonies such as Canada, New York, and New Jersey as well as in the South after the fall of Savannah. Provincial forces had regular pay, uniforms, and more prestige than a militia, though not as much as a Regular British regiment of course. They also became a big back bone for the British army in the Southern Campaign after the Siege of Charlestown in 1780.

The British army recognized the recruiting potential of Americans all the way back in the French and Indian War. Raising American forces did not cost as much and would allow faster movement across territories than shipping regiments from Britain. The British would repeat this in the American War for Independence and began doing so in summer of 1775 when the War Office realized what kind of army they would need to combat the rebellion. Almost every colony or region provided at least one Provincial regiment or militia while some such as New York gave the British the recruiting grounds for more than a handful.

Provincial forces served almost as a last ditch resort of promotion for some or the career opportunities Regular army could not easily provide. Some British officers left Regular regiments as middling ranks to be commanders of regiments in the Provincial forces. Banastre Tarleton would be a notable example along with John Graves Simcoe. In addition to officers, some non commissioned officers in the British army also saw opportunities in the Provincial army for opportunities in advancement. One step back for a couple steps forwards in some of these cases. While the Provincial forces at first did not have the same benefits as Regular army, a commission of a rank was still worth the same pay as one in the British establishment.

The British army did pull recruits from the colonies for their Regular regiments but not nearly as much as they did from the British isles (and Ireland). As the war went on for more than a few years and the British suffered losses the army needed to replenish their strength and did so with local populations. Willing ones I want to add, they did not use conscription as the bases of recruitment. The rare exception would be using prisoners of war, but British commanders and leaders heavily decided against it and did not see the benefits as a major source of recruits. Correctly, they saw most of them as potential deserters than soldiers. At most regiments pulled a dozen or so from local areas, not full companies usually.

So Loyalists had a few options when it came to enlisting into the British army from a county militia to the rare chance of a Regular British regiment, but most often joined a Provincial regiment.