How to handle auto-backstab on my rogue party members?

There are several ways in which you can backstab an opponent.

1. You are attacking him from behind.
2. The opponent is stunned.
3. The opponent is frozen.

So if you manage to stun the enemy very often using your skills or your team mate's skills, then you'll also be able to get the bonus backstab damage without attacking from the back half circle.

Manually positioning the rogue is must if you want to backstab from the back half circle, as there is no way you can alter the tactics to make the rogue position himself/herself behind the enemies. Although one thing that is worth noting is that you need to have Coup-De-Grace for easier backstabbing.

Backstabs in DAO occur when a rogue attacks anywhere from the back half of the circle of the enemy.

Since you do not want to manually control the rogue, then the only thing you can do is control the target's positioning.

If you are a warrior, you can use skills like taunt to turn enemies so their back is exposed to the rogue. Other than that, manually controlling the rogue is the best way to get them to backstab. Just move them to the backside of an enemy before attacking with them.