I had this same issue. As of the RSVP date over half the guests had not yet responded. I gave my sister email addresses for all of the people we hadn't heard from yet, and she sent out a mass email, just BCC'ing everyone who hadn't responded. Of about 20 we hadn't heard from, more than half ended up showing up! Her email said:
"Hello, ladies! If you are receiving this email, I have not yet heard from you regarding whether you are planning to attend Beth's baby shower on December 1st. I hope you can make it -- Beth would love to see you there! Please let me know either way, as we need to finalize our numbers for food and beverages in the next few days. And feel free to let me know if you have any questions. Thank you so much, and I hope everyone had a wonderful Thanksgiving!"