User

user.cs

public class User
{
    public Guid Id { get; set; }
    public string Name { get; set; }
    public List<Department> Departments { get; set; }
}

public class Department
{
    public Guid Id { get; set; }
    public string Name { get; set; }
}

var users = new List<User>();
users.Add(new User
{
    Departments = new List<Department>
    {
        new Department
        {
            Name = "Dept 1"
        },
        new Department
        {
            Name = "Dept 2"
        },
    },
    Name = "French Coding",
});

users.Add(new User
{
    Departments = new List<Department>
    {
        new Department
        {
            Name = "Dept 2"
        },
        new Department
        {
            Name = "Dept 3"
        },
        new Department
        {
            Name = "Dept 4"
        },
    },
    Name = "Homme Poux",
});

users.Add(new User
{
    Departments = new List<Department>
    {
        new Department
        {
            Name = "Dept 1"
        },
        new Department
        {
            Name = "Dept 4"
        },
    },
    Name = "Pascal Paradis",
});