Deadheading pilots, in uniform, can board at anytime, as long as the FAA minimum compliment of F/As are onboard. Out of uniform, they can board at anytime during the passenger boarding process. I don't know the specifics for deadheading F/As but they seem to be similar.
Pilot jumpseaters can board anytime, as long as at least one of the working pilots is onboard.
Non-revs (not deadheading) board in the appropriate group for the cabin in which they are seated. If the flight is relatively full, we often won't get our seat assignment until after our boarding group has boarded. The automatic sweep of the standby list is at T-30, the check-in deadline for Domestic, but on full-ish flights, at least some of the non-revs may have to wait until the end of boarding to see if there are enough no-shows for them to get a seat.