There's no requirement that the guest(s) be on your flight, flying DL, or even flying if they can get through security to the SC. I've never seen a lounge dragon ask guests for boarding passes, although I think I have seen them verify that more than two guests are family members.

I've also never seen enforcement of the "must stay with guests" rule. I usually find seats and then show new guests (or point in the general direction) the lavatories, the food and beverges, mention that it's free (and that some folks tip if there's a bartender), etc. but I do not walk around the lounge with them to get F&B and I certainly don't take them to the restrooms.

I've sometimes gone to catch an earlier flight and left my guests in the SC (although after they leave, they would not be able to return to the lounge). However, doing this might be obvious at a small club or one with very few people. Under the current crowded conditions, dragons usually have no means or reason to remember who brought guests unless something calls attention to the situation.

However, I always consider myself responsible for the behavior of guests. In particular, I ask guests to be considerate and I would not invite people likely to be loud and rowdy or someone who isn't appropriately dressed (which isn't a very high standard, but I can imagine some extreme examples).