Well, since you actually don't want Left4Dead style spawnpoints but simple multiplayer spawnpoints, you can use something like that:

// C#
// SpawnPoint.cs
using UnityEngine;
using System.Collections.Generic;
using System.Linq;

public enum Team
{
    Survivor,
    Infected
}

public class SpawnPoint : MonoBehaviour
{
    // players should be on custom layer 8 so they can be
    // detected if they block a spawn point
    private static int m_PlayerLayer = 8;

    // range around the spandpoint which should be empty to use this spawn point
    private static float m_ClearRadius = 2.0f;

    // Set in the inspector
    public Team team = Team.Survivor;

    public bool IsThisPointClear()
    {
        var players = Physics.OverlapSphere(transform.position, m_ClearRadius, 1 << m_PlayerLayer);
        return players.Length <= 0;
    }
    public static SpawnPoint FindFreeSpawnPoint(Team aTeam)
    {
        var spawns = FindObjectsOfType(typeof(SpawnPoint)).Cast<SpawnPoint>().Where(O => O.team == aTeam).ToList();
        while (spawns.Count > 0)
        {
            int i = Random.Range(0,spawns.Count);
            if (spawns[i].IsThisPointClear())
                return spawns[i];
            spawns.RemoveAt(i);
        }
        return null;
    }
}

Just place some empty gameobjects in the scene, attach this script and select the team for each spawnpoint.

Now you can simply use this when ever you need a spawn point:

var SP = SpawnPoint.FindFreeSpawnPoint(Team.Survivor);
if (SP == null)
{
    Debug.LogError("All spawns occupied and / or not enough spawn points");
    return;
}
Instantiate(playerPrefab, SP.transform.position, SP.transform.rotation);