Droppables

If you need just to test unit or set some neutral creeps on the map you need to create blueprint and set it inherited form needed basic class. In our case, it is the Unit class.

Open up blueprint and click on the EntityComponent.

Here you need to set EntityConfig in settings. Also you can set team. By default all neutral units have 66 team index (to be nice and friendly to each other).
Click at the Skeletal Mesh and here you can set some preview mesh for convenience. But in-game it will be replaced by mesh that was specified in the EntityConfig

After that you'll be able just to drop blueprint on the map. Unit will create all other stuff like Handle and set itself PlayerController and then assign it to neutral team by default. If you produce your own units in a production building, you'll be able to attack this spawnable unit

*Group size is also working in this option. If you set 6 as a group size, when the game starts, the other 5 units will be spawned at the location of the first unit. So at first, you might not notice other units as they all will be at one exact spot. You can set sentry tactic in blueprint settings and all units will start move around the specified distance.