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It seems there is not any built-in function to do what you want, but you can write your own function to do what you want. However, be careful, the function is quite slow, so do not use it frequently :

using System.Collections.Generic ;

private class PlayersManagerByLayer
{    
    private Dictionary<int,List<GameObject>> gameObjectsByLayer ;

    public PlayersManagerByLayer()
    {
        gameObjectsByLayer = new Dictionary<int,List<GameObject>>();
    }

    public void AddPlayer( GameObject player )
    {
        if( !gameObjectsByLayer.ContainsKey( player.layer ) )
            gameObjectsByLayer.Add( player.layer, new List<GameObject>() );

        if( !gameObjectsByLayer[player.layer].Contains(player ) )
            gameObjectsByLayer[player.layer].Add( player ) ;
    } 

    public void RemovePlayer( GameObject player )
    {
        if( gameObjectsByLayer.ContainsKey( player.layer ) && gameObjectsByLayer[player.layer].Contains(player ) )
            gameObjectsByLayer[player.layer].Remove( player ) ;
    }

    public void MovePlayerToLayer( GameObject player, int newLayer )
    {
        RemovePlayer( player ) ;
        player.layer = newLayer ;
        AddPlayer( player ) ;
    }

    public List<GameObject> GetPlayersFromLayer( int layer )
    {
        if( !gameObjectsByLayer.ContainsKey( layer ) )
            return null ;
        else
            return gameObjectsByLayer[layer] ;
    }

}

It seems there is not any built-in function to do what you want, but you can write your own function to find every object in a given layer. However, be careful, the function is quite slow, so do not use it frequently :

using UnityEngine ;
using System.Collections.Generic ;

private class PlayersManagerByLayer
{    
    private Dictionary<int,List<GameObject>> gameObjectsByLayer ;

    public PlayersManagerByLayer()
    {
        gameObjectsByLayer = new Dictionary<int,List<GameObject>>();
    }

    public void AddPlayer( GameObject player )
    {
        if( !gameObjectsByLayer.ContainsKey( player.layer ) )
            gameObjectsByLayer.Add( player.layer, new List<GameObject>() );

        if( !gameObjectsByLayer[player.layer].Contains(player ) )
            gameObjectsByLayer[player.layer].Add( player ) ;
    } 

    public void RemovePlayer( GameObject player )
    {
        if( gameObjectsByLayer.ContainsKey( player.layer ) && gameObjectsByLayer[player.layer].Contains(player ) )
            gameObjectsByLayer[player.layer].Remove( player ) ;
    }

    public void MovePlayerToLayer( GameObject player, int newLayer )
    {
        RemovePlayer( player ) ;
        player.layer = newLayer ;
        AddPlayer( player ) ;
    }

    public List<GameObject> GetPlayersFromLayer( int layer )
    {
        if( !gameObjectsByLayer.ContainsKey( layer ) )
            return null ;
        else
            return gameObjectsByLayer[layer] ;
    }

}

GameObject[] FindGameObjectsInLayer(int layer)
{
    GameObject[] goArray = FindObjectsOfType(typeof(GameObject)) as GameObject[];
    System.Collections.Generic.List goList = new System.Collections.Generic.List();
    for (int i = 0; i < goArray.Length; i++)
    {
        if (goArray[i].layer == layer)
        {
            goList.Add(goArray[i]);
        }
    }
    if (goList.Count == 0)
    {
        return null;
    }
    return goList.ToArray();
}

GameObject[] FindGameObjectsInLayer(int layer)
{
    var goArray = FindObjectsOfType(typeof(GameObject)) as GameObject[];
    var goList = new System.Collections.Generic.List<GameObject>();
    for (int i = 0; i < goArray.Length; i++)
    {
        if (goArray[i].layer == layer)
        {
            goList.Add(goArray[i]);
        }
    }
    if (goList.Count == 0)
    {
        return null;
    }
    return goList.ToArray();
}

using System.Collections.Generic ;

private class PlayersManagerByLayer
{    
    private Dictionary<int,List<GameObject>> gameObjectsByLayer ;

    public PlayersManagerByLayer()
    {
        gameObjectsByLayer = new Dictionary<int,List<GameObject>>();
    }

    public void AddPlayer( GameObject player )
    {
        if( !gameObjectsByLayer.ContainsKey( player.layer ) )
            gameObjectsByLayer.Add( player.layer, new List<GameObject>() );

        if( !gameObjectsByLayer[player.layer].Contains(player ) )
            gameObjectsByLayer[player.layer].Add( player ) ;
    } 

    public void RemovePlayer( GameObject player )
    {
        if( gameObjectsByLayer.ContainsKey( player.layer ) && gameObjectsByLayer[player.layer].Contains(player ) )
            gameObjectsByLayer[player.layer].Remove( player ) ;
    }

    public List<GameObject> GetPlayersFromLayer( int layer )
    {
        if( !gameObjectsByLayer.ContainsKey( layer ) )
            return null ;
        else
            return gameObjectsByLayer[layer] ;
    }

}

using System.Collections.Generic ;

private class PlayersManagerByLayer
{    
    private Dictionary<int,List<GameObject>> gameObjectsByLayer ;

    public PlayersManagerByLayer()
    {
        gameObjectsByLayer = new Dictionary<int,List<GameObject>>();
    }

    public void AddPlayer( GameObject player )
    {
        if( !gameObjectsByLayer.ContainsKey( player.layer ) )
            gameObjectsByLayer.Add( player.layer, new List<GameObject>() );

        if( !gameObjectsByLayer[player.layer].Contains(player ) )
            gameObjectsByLayer[player.layer].Add( player ) ;
    } 

    public void RemovePlayer( GameObject player )
    {
        if( gameObjectsByLayer.ContainsKey( player.layer ) && gameObjectsByLayer[player.layer].Contains(player ) )
            gameObjectsByLayer[player.layer].Remove( player ) ;
    }

    public void MovePlayerToLayer( GameObject player, int newLayer )
    {
        RemovePlayer( player ) ;
        player.layer = newLayer ;
        AddPlayer( player ) ;
    }

    public List<GameObject> GetPlayersFromLayer( int layer )
    {
        if( !gameObjectsByLayer.ContainsKey( layer ) )
            return null ;
        else
            return gameObjectsByLayer[layer] ;
    }

}