EntityBuffer.cs

using Unity.Entities;

[InternalBufferCapacity(4)]
public struct EntityBuffer : IBufferElementData
{
    public Entity entity;
}

FollowTargets.cs

using Unity.Entities;
using UnityEngine;

public class FollowTargets : MonoBehaviour
{
    [SerializeField] GameObjectEntity[] entities;

	void Start ()
    {
        EntityManager entityManager = World.Active.GetExistingManager<EntityManager>();
        Entity entity = GetComponent<GameObjectEntity>().Entity;
        entityManager.AddBuffer<EntityBuffer>(entity);

        DynamicBuffer<EntityBuffer> buffer = entityManager.GetBuffer<EntityBuffer>(entity);

        for (int i=0; i<entities.Length; i++)
        {
            buffer.Add( new EntityBuffer() { entity = entities[i].Entity });
        }
	}
}