using System;

using System.Collections;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var userIds = new int[] {1, 2,3,5};

        var tenantUsers = new List<TenantUser> { new TenantUser { UserId = 1, User = new User {Id = 1}, TenantId = 1, Tenant = new Tenant { Id = 1}} };

        var tenants = tenantUsers

        .Where(m => userIds.Contains(m.UserId))

        .ToLookup(x=>x.UserId, y=>y.Tenant); 

        IEnumerable<Tenant> tenantList = tenants[1];

        foreach(var t in tenantList)

            Console.WriteLine(t.Id);

    }   

}

​

public class User

    {

        public int Id { get; set; }

        public List<TenantUser> TenantUsers { get; set; }

    }

​

​

    public class TenantUser

    {

        public int UserId { get; set; }

        public User User { get; set; }

        public int TenantId { get; set; }

        public Tenant Tenant { get; set; }

    }

​

    public class Tenant

    {

        public int Id { get; set; }

        public string Name { get; set; }

        public List<TenantUser> TenantUsers { get; set; }

    }