mostly involving extracurricular activity and people you know somewhat rather than total strangers. asking a woman if she wants to come over to your house tonight to taste whiskey with you and another male coworker, just not quite the same. invite on a group ski trip with a reservation for only one hotel room. join me for dinner to talk about project/feature. a packed car trip to somewhere in a very small vehicle. let's go visit the hot springs
(which, it's common knowledge, are clothing-optional at night).
taking that last one further, i was once told it was ok that i had forgot my swimsuit, for that reason. man to man, i thought nothing of it, but by god could you imagine saying that to your female coworker?
none of these things are crimes, but there's zero chance anyone would ever read anything into it or that anything would happen if it's a bunch of guys, while there's clear places a woman might hear a different meaning or have reservations, hence the newfound question marks.
anyway, i'm not the canonical source on this, just attempting to think of my own experiences working in a boy's club.
and of course if a man is truly close & open friends, not just work acquaintances, with a woman, pretty much everything said here may go out the window.