Hi,
I'm using the Thieves Guild Shortcut (v1.1 which doesn't require a key) which adds a trap door between Riften and the Ragged Flagon, but my follower can't use it. I remember having the same problem in Oblivion, but can't remember the solution.
Thanks for any help.

Have you finalized the navMesh of both cells containing the door and it's target (and the tri's turn green)? Did you assign faction ownership to it (which may prevent, not allow)?

I didn't make the mod, but thanks, I'll look into it.

You might also want to let the mod author know if you haven't already so that he can try to fix this issue himself.

I'll let the mod author know as soon as I know the solution, but it still escapes me. Can anyone who has added a door which followers can use please help me in the right direction (a tutorial maybe), I've searched the creation kit wiki but come up at a loss ? Thanks for any help !

It's almost certainly that the Navmesh isn't finalized.

I agree with everyone above ...

1. Make a navmesh for the interior
2. Make sure the GREEN TRIANGLE from the door-teleport box/marker is sitting nicely on that navmesh (interior and exterior)
3. Finalise the interior navmesh
4. Depending on how much work you did to the exterior you may need to finalise that navmesh - BUT DON'T DO IT IF YOU ONLY ADDED A TRAP DOOR! (Exterior navmesh is one of the dark-arts ...)

Yep, that was it, thanks for the help!