The party has gotten a (home-brewed) sentient Construct in its possession that acts like a vehicle for the party, carrying them inside of it. The question we stumbled into is:
What happens if a party-member casts Invisibility on it?
Do the people inside of the Construct also share the Invisible condition?
From the rules for the spell invisibility:
A creature you touch becomes invisible until the spell ends. Anything the target is wearing or carrying is invisible as long as it is on the target's person. The spell ends for a target that attacks or casts a spell.
If you are inside of the construct, then it is carrying you. The rules clearly state that anything it is carrying is also invisible.
The spell invisibility reads (emphasis mine):
A creature you touch becomes invisible until the spell ends. Anything the target is wearing or carrying is invisible as long as it is on the target's person. The spell ends for a target that attacks or casts a spell.
A creature is not an object/thing; it is a creature. If I said "anything in the house has to be washed down with bleach" or "anything left in the house when you sell it becomes the property of the new owner," you wouldn't expect people to be included because they aren't things.
While we would agree that objects in a target's pocket or backpack are "on the target's person," I don't think counting a creature riding inside a construct as "on the person" of the construct is a bit of leap/stretch. I don't think if I picked my daughter up to carry her, I wouldn't use the phrase "on my person" the same way I would talk about my wallet or keys.
Jeremy Crawford tweeted about a similar abuse - having a Warlock's familiar carry them to share their invisibility. He said:
@darthmallcore "Can pact familiar imps and quasits turn their masters invisible?"
Nope, that invisibility works on the fiend and its gear.
At the time he made the post, his tweets were considered "official rulings" although that has since changed. This doesn't, however, change the fact that his tweet is an insight into the designer's intent.
In answer to another question about picking things up after invisibility was cast, he said:
Only items worn/carried when invisibility is cast are invisible, but I'd let you conceal something under them. #DnD
Casting invisibility on the construct takes only the base level slot (2nd). Normally, casting it on more people would require a higher level cast. If casting it on the construct with people inside makes them invisible as well it is like giving you free spell slots.
At Higher Levels. When you cast this spell using a spell slot of 3rd level or higher, you can target one additional creature for each slot level above 2nd.
Moreover, if it is cast only on the construct then the party members would be able to attack while invisible without breaking their invisibility since the attack clause only mentions the target of the spell. This makes greater invisibility (4th level) moot.
The spell ends for a target that attacks or casts a spell.
