Sometimes you can forget to include all the guests while sharing your RSVP invitation. But do not worry, as you are able to see the list of recipients and add new people there.
Find your RSVP invitation in the My Creations tab:
Please keep in mind that the status of the creation is already Sent.
I would like to remind you that here you can check the replies from the recipients: 
But if you want to add more guests, please click the button "Add Guest".
Enter the name and email of the new recipients, and confirm them with the plus icon:
Click the button Send and check the confirmation:
Please do not worry, your previous recipients will not get the invitation again. Only the new recipients will get the email from you.
Enjoy, and remember that if you have any questions, just contact us.