Depends.
If you're going on a mission and the tanks are already throwing away 30k+ for their consumables so that they have a reasonable chance of surviving, *cough* Riverne *cough* help them out and take care of their sneak/invis ESPECIALLY if it's just one or the other. This goes doubly for NINs (and RNG) since they're already throwing gil at the enemy.
If it's an EXP party and there's more than one mage that can sneak/invis, split the pty in half and move everyone that way. If you only have one mage, yeah, people are going to need to bring their own powders and oils.
The way I see it, this game is expensive enough gil-wise. If you can help somone else save money, do it.
If you've got an issue with using sneak/invis, just /tell "you know, I don't want to risk dying due to aggro," or "I'm sorry, I can't sneak/invis this many at once," and make sure the pty knows that before they leave the vicinity of an AH. Don't scream in party chat "I'M NOT YOUR DAMNED SNEAK/INVIS WHORE!!!" when you're halfway to your destination.