1. Skypriority will not be on their boarding passes even if under the same reservation number.
2. U could use Skypriority with them including priority check in or baggage handling. But, as you know, only one companion to the lounge.
p.s. Sometimes, at ICN, entry to Premium check-in Zone A can be quite strict. They usually allow up to one or two companions under same reservation , but I've seen some cases that they restrict entry to larger groups.