You only require line of sight to the target (not line of effect) and it hason its to resist the spell if you or your companions are fighting it. Until the spell ends, the target is by you and friendly towards you.
The spell ends if you or your companions do anything harmful towards the target. The target knows it was charmed by you when the spell ends.
For each slot level above 4th, you affect one additional target that is within 30 feet of other targets.