That task can be a little wonky sometimes. When it works correctly, they should board your vehicle automatically (make sure you are using a vehicle with enough space for them to board!).
But I have witnessed it fail during certain circumstances, especially when their pickup point is under enemy contact. I've also seen this portion of the task successfully complete without the unit boarding as well (why I don't know).
It's inconsistent enough where if I use true auto-tasking, I disable the task completely. However I still periodically request it when I want to mix things up.
If a task is failing, during the game you can delete an active task through through the commander tablet. If you want to disable the task entirely, add a staticdata.sqf file to your mission root and use this code (remove the transportinsertion array):