Because I realized I had need for something like this, both through play and through chatting with someone outside my group, here are rules for handling reinforcements when you fight part of a group of monsters inside their lair.
Hue and Cry: Not all monsters will be in their lair at all times. While this makes it easier to fight a small group, the others may show up and fight later! When fighting some monsters in their indoor lair, make a Per or Per-based Observation roll for the monsters; add any bonuses for Acute Hearing or similar Hearing-based traits. On a success, a number of monsters equal to one plus the margin of success will show up in a minute, and they will be aware there is fighting. Any not showing up in this group will show up in 1d hours, and will be unaware there is a fight. On a critical success, that number of monsters will show up in 3d seconds, with any not coming in this group will show up in a minute; all will be aware there is fighting. On a failure, all of the monsters will show up in 1d hours, unaware of anything amiss.
Outdoors, folks are usually farther away, leading to a lower chance of someone being around to hear the fight. Faster monsters wander farther than slower ones. Thus, take the lowest encumbered Move of the monsters, not including any bonuses for Enhanced Move, and look it up in yards on the Speed/Range Table (p. B550). Apply twofold this as a penalty to any Hearing-based rolls as above; don't apply a bonus for Move less than 2.
Roll Per or Per-based Observation as for indoors, with the penalty above for distance. On a success, half of any monsters out wandering will show up in 1d/2 hours (keep fractions), and will be aware there is trouble. The other half will show up in 1d hours, unaware of anything amiss. On a critical success, half the monsters will show up in 3d minutes, and the other half will show up in 1d/2 hours (keep fractions), all aware of the fight. On a failure, nobody will be aware of the fight; half will show up in 1d hours, as will the other half (roll separately for each half). Divide any times for monsters who are aware of trouble and using Enhanced Move to fight by one plus the level of Enhanced Move.
Signaling: Any monsters outdoors may try to signal their fellows out wandering. Use the rules for Signaling on p. 28 of GURPS Dungeon Fantasy 16: Wilderness Adventures. Assume that each half of those monsters are 1d times Move/2 miles away; roll separately for each half.
Either indoors or outdoors, if the delvers start fighting any reinforcements, make another Per or Per-based Observation roll for anyone else still not in the room or lair. Handle as above, except this goes for all monsters that would show up as reinforcements, not half or margin of success plus one. This keeps the GM from having to roll for smaller and smaller groups!
The upshot of all this is that players never should be sure they have cleared the room or lair, which is as it should be.