Charm Monster
One creature you can see within range makes a Wisdom saving throw. It does so with Advantage if you or your allies are fighting it. On a failed save, the target has the Charmed condition until the spell ends or until you or your allies damage it. The Charmed creature is Friendly to you. When the spell ends, the target knows it was Charmed by you.
Using a Higher-Level Spell Slot. You can target one additional creature for each spell slot level above 4.
Charm Monster 5.5e 2024 vs. 5e 2014 edition differences
The 2024 version states the creature makes a Wisdom save, with advantage if you or your allies are fighting, and is charmed until the spell ends or you/your allies damage it, whereas the 2014 version says the creature makes a save with advantage if you or your companions are fighting, and is charmed until you or your companions do anything harmful. The 2024 version clarifies the target makes the save, and the condition is explicitly "Charmed." The higher-level casting effect is simplified, allowing targeting of one additional creature per spell slot level above 4, without specifying the 30-foot range for additional targets.