jQuery Validation Plugin - adding rules that apply to multiple fields

I'm using the jQuery Validation plugin:

I have several fields in a large form that I want to apply the same rules to. I've simplified the code for this example. This code doesn't work, but I want to do something like this. The second rule should apply to both first_name and last_name. I want to encapsulate all the rules here in this function and not edit the class lists of some of the form elements to add a required class or whatever.

(Again, I have several different groups of form fields with different requirements that I want to group. I only put required: true in that element for simplicity)

$('#affiliate_signup').validate(
       {
          rules: 
          {
            email: {
                required: true,
                email: true
            },
            first_name,last_name: {
                required: true
            },
            password: {
                required: true,
                minlength: 4
            }
          }
       });

Thanks in advance.