let players = []; // define array
const player = runtime.objects.player.getFirstInstance(); // get the player instance you want to put into the array. In this case it will always get the first instance of player which is fine if there's only one player. Otherwise you'll have to get it via uid or loop through all player objects.
players.push(player);
Your array now contains the player instance with all it's properties, instance variables etc.
You can show it in the console via a simple console.log(players)