I have an assist key made from the macros editor.
-Open the socials window (press the O key)
-click the Macros tab
-click Edit
-in the command line type /assist <name of main tank> (without the <'s)
-select an icon
-give the macro a name
-drag the new macro to your hot key bar
When joining a new group, right-click the key and select edit to put the MT's name and you're all set.
also F8 targets nearest npc..but I've noticed it also targets resource nodes and other stuff as well. I was tanking for a group doing the gnoll tormenators in front of BB...hit F8 for the next mob but targeted the tent instead...had me confused for a moment, until one of the elders smacked some sense into me, and the fight went on.