The party command for targeting is just the #. So, for assist, you'd use "/assist <2>" and it would target the mob that party member 2 is fighting. You may rearrange your party list using the party menu. Also, it can help to mark a target as well. To mark, use the /mk command (for example, "/mk attack1" would mark whatever you're targeting as the attack 1 sign). You can combo these together as well to mark the tank's attack target as attack 1 and everyone in the party can see. Hope this helps!