im guessing you dont know how to make an array variable. or are you asking how to put actors into the array.
ti make an array you go to the my blueprints area in the bottom left and click the + sign next to the variables heading. then select the variable and go to the details panel and look for the symbol next to the variable type. click the symbol there (it should be a capsule like shape) and change it to the one with all the boxes.
to set the actors in the array you have a few options. you can set the variable to be editable so when the enemy is placed in the level you just go to the details panel and fill in the array. Or you could use the make array node to set the array. Or you could set the value from a list when the enemy is spawned. or you could use a get all of class node to set the values though this method wouldnt allow control of the order or length of the array. theres many ways to accomplish this task. without knowing the full exent of what your looking to do its hard to say which method is the right one to use.