Just as with hosting platforms, there are various methods for inviting guests to a virtual baby shower, too.
A Facebook event allows you to share all of the necessary information and have guests RSVP on a platform most of your guests are probably already using. You can easily upload a link to the new parent's baby registry and any games you want to play during the shower.
Paperless Post, Minted, and Evite all offer tons of beautiful online invitations (bonus: you can help save a tree or two) and allow you to easily track RSVPs. After the shower, you can use these to send thank-you cards, as well.