C# - ViewModel Validation For A List?

I have the following viewmodel definition

public class AccessRequestViewModel
{
public Request Request { get; private set; }
public SelectList Buildings { get; private set; }
public List<Person> Persons { get; private set; }
}

So in my application there must be at least 1 person for an access request. What approach might you use to validate? I don't want this validation to happen in my controller which would be simple to do. Is the only choice a custom validation attribute?

Edit: Currently performing this validation with FluentValidation (nice library!)

RuleFor(vm => vm.Persons)
.Must((vm, person) => person.Count > 0)
.WithMessage("At least one person is required");