pmm711: If someone drops from a multi-player game then just the person who drops loses their XPs. The player(s) remaining still earn their XPs if they finish....at least in CTTH, Blitz, Match Play, & Skins.
My experience is different:
In a match or A/S, there is no finish (and no win, @pmm) whenever one party leaves, so you'd totally miss the completion XPs. Only stroke XPs and (newly) CDP XPs have been awarded and are not affected.
In those games where a solo finish is possible, like stroke play, you'd finish under "solo" conditions, so you'd lose any extra XPs provided for the MP experience.