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I've wondered about this. A notable example is:
- The Magician's Nephew: the magic rings which transport people to or from the Wood between the Worlds work if somebody is touching somebody who is touching the ring. Polly and Digory find this out when they accidentally bring the witch Jadis back with them, and they later use it to try to return Jadis to her own world.
I listed it under Accidental Passenger, but it could be a trope in itself.
Checking Teleportation Tropes gives following fits:
- Mass Teleportation: duh
- No-Flow Portal: Since the portal turns out to be very selective on how it works
- Teleporter Accident: both of your examples are about accidental situations
^^ Ah, I searched for teleportation passenger but I didn't try "stowaway".
^^^ Definitely some relation, but none of those really covers it fully. And I wouldn't really call the Death Eater example an accident; he was grabbing her because he was trying to catch her and not let her get away, following her through Apparition is just a logical continuation of that.
We have Power Rangers, Star Trek IV: The Voyage Home, Harry Potter and the Deathly Hallows and The Magician's Nephew - that's four examples. Let's do it! (BTW, What a Drag is about physical dragging, so no, not related. But Accidental Passenger definitely is related).
"Teleport Stowaway" sounds like a good name for this trope - it's informative and to the point.

When a person teleports away, anyone holding on to them will come along for the ride — with or without the teleporter's permission, or even knowledge.
Eg in Harry Potter and the Deathly Hallows, Hermione Apparates to Grimmauld Place to escape, but a Death Eater manages to grab her and be pulled along, thus being granted access to the secret of its location.
It seems somewhat related to What a Drag, but that one is about intentionally dragging someone behind you as a punishment. There could sometimes be overlap if being pulled into a teleport is harmful, but What a Drag doesn't cover the case above.