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 They can follow an NPC that is on a grid, they just can't be assigned to a grid themselves (the followers), which also includes a temporary grid.  I think even if you use the quest::moveto command and try to make them follow something while they are on the way to that point still, it will fail because essentially it is treated like a grid with a single point in it.
 Maybe you should post the whole script you are trying to make for one of these NPCs.  It should be fairly easy to get it working.
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