Anyone can use the /assist function, regardless of job. For example, if you target the mob that is attacking anyone (even not in your group) you can /assist the mob, to then target the player its attacking for a quick heal (usefull in Fates).
In dungeons, I primarily leave the tank as my target, then instead of assisting him, if you use the auto target (tab for kb, or on a controller the arrow keys), you can cast One spell on the reticule target, and it will automatically reapply back to your main target (the tank).
Example: I target the tank for heals... want to throw Aero on the mob, press right arrow key in my case, cast Aero, and it switches back to the tank whiles its casting, then I can cast heal right away.
Rinse repeat as needed.
____________________________ Things I sometimes play...
"What do you want to be when you grow up?"
"I want to be a unicorn!"
"Awww, why's that?"........
"So I can stab people with my face."