Here's the relevant section from the DMG (p 37):
Only characters who take part in an encounter should gain the commensurate awards. Characters who died before the encounter took place, or did not participate for some other reason, earn nothing, even if they are raised or healed later on.
and...
4. Divide the base XP award by the number of characters in the party. This is the amount of XP that one character receives for helping defeat that monster.
This is, of course, a bit vague. Obviously, we can say that the moody swashbuckler who was upset at being insulted by the gruff dwarf and decided to sit out the combat in a sulk does not get any xp. But what about the wizard who steps up into the fray (say as a double move) and then gets hit on the head by an orc and dropped to -1, stabilizes the next round and then sits out the rest of the combat as his colleagues are too busy to get him healed? Does this equate to
helping defeat the monster?
I guess it depends on how you interprate
even if they are raised or healed later on. On one level this does seem to imply that the wizard would receive nothing, but perhaps it may also have been referring to someone who was previously injured or killed (perhaps some reinforcements arrived several rounds after the first combat, for example, and a new combat ensued).
My interpretation is that I always award full XP to everyone invloved in the combat, irrespective of how long they were involved in the actual combat itself. In the above example, for instance, did the wizard help
defeat the monster? Well, the orc had to move to the wizard and hit him, which (perhaps) brought him into charging distance for the gruff dwarf, who perhaps critted and took out the orc... and so on. And even if the dwarf in fact missed, the orc had still used up all or part of his action on the wizard. It just keeps things simple, fair and consistant.
YMMV of course.