Besides guild events, there are two modes when most people are not running solo - Lava/Mesa and WG/IS. And while there is a random PUG feature for both, most people AFAIK are running in stable groups. Every other day at same time the group gets together and runs both Lava and IS.
Except that sometimes one group member can't make it, so the rest of group looks for a substitute, either in guild or in some common chat rooms. "Hey, can someone fill in for 8 runs at P3 or P4?"
And that works well for lava, because most of us have plenty of spare lava cards from quests. IS cards, not so much. No mode drops those as far as I can see. That means subs can't run IS without messing up their usual groups. So, whether I am a sub or looking for a sub, I rarely expect to be able to get IS runs.
So.... can IS cards be added to some common mode? AFAIK, no mode gives you those.
Name: Rusty (Rustyx in game)
Guild title: resident wisenheimer.
Line ID: Rustytroll (big shock there, eh?)