If you're going into a combat arms unit I'd presume it's an MTOE unit, (as opposed to TDA). In such a case, they get called up as units. So if you're in 1-221 Cav, for example (My old unit), when 1-221 Cav gets called up in the rotation, everyone goes. If they are short-handed, they may ask for volunteers, if they're substantially short-handed, you'll be voluntold. (As in my old, old unit, we got mobilised to provide a company to fill out an undestrength battalion in another brigade).
Last I heard, the cycle was that every five years, a Guard unit becomes available to Big Army. The Army may or may not choose to call upon it, depending on what is going on in the world at that point. If it does not get called up within that year, the unit resets, and starts the four year spin-up process again.
TDA units tend to be those required for State/US-specific missions, such as State HQ, training institutes, and so on. They generally do not deploy.