You might have more success finding an answer to this question by being more specific. The medieval world was a gigantic and diverse place, and medieval society was around for a long time.

If you narrowed down by location and time frame, and clarified what you meant by "guard" (what duties are you thinking of in particular? What people or objects or places are under guard here? A household guard in the court of King Henry II was probably involved in different activities than a sentry in a large city in 1500) then I bet one of the many awesome medieval historians here could give you a great answer!

This is a bit of heavy reading, but probably is more than enough for you to digest. Bit later period, though. It details the construction and makeup of the urban militia, mostly of the major Free Imperial Cities. http://actaperiodica.org/documents/APD2013/APD1_J_Chandler.pdf

I have a book buried here somewhere that details the basics of the Night Watch / police in Rothenberg ob der Tauber, where the citizens of each district took turns manning sections of the city walls their district was responsible for, and conducting patrol throughout their district at night. In this way, all shared the tasks. There might be some paid "professional" guardsmen, but not a sizeable number.

So while it wasn't exactly a draft, everyone who was actually a CITIZEN was given basic weapons training and expected to take their turn on watch.

If i find the book, I'll edit for source.