Use tags to automatically add spawned GameObjects to the list of followers-of-the-leader.

TaggedLeaderFollow.cs

using System;
using System.Collections.Generic;
using Pathfinding;
using UnityEngine;

namespace BehaviorDesigner.Runtime.Tasks.Movement.AstarPathfindingProject
{
    [TaskDescription("Tagged GameObjects Follow the leader with the A* Pathfinding Project.")]
    [TaskCategory("Movement/A* Pathfinding Project")]
    [TaskIcon("Assets/Behavior Designer Movement/Editor/Icons/{SkinColor}LeaderFollowIcon.png")]
    class TaggedLeaderFollow : LeaderFollow
    {
        [Tooltip("Tag to generate a list of agents")]
        public String agentTag;

        public override TaskStatus OnUpdate()
        {
            /**
             * We're going to set up agents depending on tag, if a tag is present.
             */
            if (!string.IsNullOrEmpty(agentTag))
            {
                GameObject[] tagged = GameObject.FindGameObjectsWithTag(agentTag);
                if (AnyNewAgents(tagged))
                {
                    agents = new SharedGameObject[tagged.Length];
                    for (int i = 0; i < tagged.Length; i++)
                    {
                        agents[i] = tagged[i];
                    }

                    aStarAgents = new IAstarAI[agents.Length];
                    transforms = new Transform[agents.Length];

                    // Set the speed and turning speed of all of the agents
                    for (int i = 0; i < agents.Length; ++i)
                    {
                        aStarAgents[i] = agents[i].Value.GetComponent<IAstarAI>();
                        transforms[i] = agents[i].Value.transform;

                        aStarAgents[i].maxSpeed = speed.Value;
                    }
                }
            }

            return base.OnUpdate();
        }

        private bool AnyNewAgents(GameObject[] tagged)
        {
            //  Short circuit the obvious and common case.
            if (tagged.Length > agents.Length) return true;

            bool hasAll = true;

            HashSet<int> knownAgents = new HashSet<int>();
            foreach (var agent in agents)
            {
                knownAgents.Add(agent.Value.GetInstanceID());
            }

            foreach (var obj in tagged)
            {
                hasAll = hasAll && knownAgents.Contains(obj.GetInstanceID());
            }

            return !hasAll;
        }
    }
}