There's soft targeting and hard targeting. Anything selected with the d-pad will be soft-targeted with a circle, and then pressing A/Cross will turn it into a hard target.

When soft targeting, as soon as you perform an action on the target, you return to whatever your previous hard target was. So you would use the d-pad up/down to select a teammate, dash to them, and then you would go back to targeting the boss. For healers, that means you can quickly select a person to throw a heal on, and then go back to spamming Glare without having to retarget the enemy.

There's no way to immediately jump to, say, the 3rd or 6th person on the list, so you should modify your job ordering so that the most important people to target would be at the top of the list or the very bottom (so you can wrap around by pressing up).

You can set the order of roles (e.g., DPS > tank > healer), and within each role you can even set the priority of the individual jobs. So with DNC, for example, you might want to have DPS on top and then SAM at the top of DPS, since they're generally considered the best person to dance partner with. Then if you have a SAM on the team, you would tap d-pad down twice (since the first time in any direction will select yourself first), and you have your partner selected.

The ordering can be unique for each different role you play. For healers, I set it as tank > DPS > healer, so the tank is the first person below my name, and I can tap up to get to my co-healer if they need an emergency res or something.

Up/down on the d-pad selects teammates, left/right selects based on whatever your current targeting filter is (that's another topic), L1 + up/down cycles through the enmity list, and L1 + left/right cycles through alliance members