can be enabled/disabled via Interface -> Game -> Mouse -> [X] Click-to-move, or thru macros:

/console autointeract 1 enables ctm
/console autointeract 0 disables ctm

together with the "Interact With Target" keybinding (should be keybound in-game under Options -> Key Bindings -> Targeting Functions -> Interact With Target), the usual scenario plays out like this for me:

im at an inn, i want all toons to talk to the innkeeper. i target the npc with my current main and have the other toons assist on it.
(just as a side note, im using a click-infused FTL system where i have a separate "/assist ToonName" macro for each, all that was required of me was to keybind these macros)
i enable ctm, then press my IWT button and everyone walks over to the innkeeper and opens a dialogue.
if you don't use the FTL system, then disable ctm otherwise you'll see toons running off to where they're not supposed to whenever the IWT button is pressed (or accidentally pressed).
like i said, i use the FTL system, and the 'targetless' aspect of this system permits me to keep ctm enabled at all times.

so yeah, in multiple melee team setups, you can use ctm+IWT to get your melee guys to hit the main's target via /assist.

ctm+IWT does have a 'timeout' btw. i mean when you don't clear an obstacle between the toon and the target, the toon will just stop on its tracks after 1 second.
and hence, in ctm+IWT, your toons will always try to go to the target in a straight line. it won't go around walls and jump over barrels/obstacles and won't turn left or turn right to get to the target. so just make sure the way is clear between your toons and the ctm+IWT target.