Originally Posted by platbrownguy

And pick your seats wisely in economy before you call, so that the one you "leave behind" is in a good (non-middle) MCE seat. 99% of the time, they will get to keep whatever MCE seat you're able to assign them for free. (The 1% is when there's an aircraft change or similar that causes reshuffling.) But they won't be able to select another premium seat for free anymore after you and your sponsored companion are split into a different PNR. Also note that the one you leave behind won't get other perks like free baggage that come with being companion of an EXP.