I assume you're using something like Bullet behavior for your enemies (as shown in the Ghost Shooter tutorial). But Bullet behavior doesn't automatically support solid collisions.
8Direction does. If you use 8Direction behavior on your sprites (be sure to set the user control to "ignored" with an event) and make them Solid, then they will automatically push out of each other if they're overlapping.
Platform, Ball, and Car behaviors also automatically handle solid collisions. Custom Movement behavior can use actions to tell it how to handle solid collisions. Physics behavior has solid collision too, but it doesn't use the Solid attribute to collide with, rather it only treats other physics objects as solid.
You could continue to use Bullet behavior, or move them yourself with your own events, but you would have to create your own collision events as well to tell your enemies to push out of each other when they overlap. But, eh... it's just easier to use 8Direction, so why not?
By the way, you'll probably want to make the collision masks for your enemies a perfect circle if you're going to use 8Direction, you'll get more natural looking crowd behavior out of them that way. If they're rectangular or irregularly shaped then they might get stuck on each other in weird ways.