Would still be stopped by it.
I agree. Nobody invites cockroaches into their house. When I said "piggyback" on someones shadow, I meant that they somehow, by extention, get an invite.
If they possessed a PERSON, and that person got invited, that would probably count as an invite!
In the middle of the night, when my toddler woke up screaming (probably from a bad dream), I couldn't help think about your shadow people. What if, when they crossed a threshold, the more powerful ones lost the ability to manifest etc...That way, in a house, they'd be very insubstantial, but maybe maintain the ability to cause fear (just as a maneuver - no actual mental damage.). So, on the whole, they're harmless but still creep people out. Outside a house, or in a hotel or apartment, they'd be able to cause much more havoc. You wouldn't have to make a custom power - maybe give them incite fear and make that the last thing to go. Or maybe it's just a mundane intimidation check. It doesn't take much to freak people out in the middle of the night.
Probably not what you're looking for, but since I was up part of the night thinking about it, I thought I'd share