After sending your invitation, you can add the "What to Bring" option by clicking on the red bag icon(screen shots below). Once you fill out your requested items, simply select "Post" and your WTB option will then appear on your invitation. Any invited guest will be able to access their invitation and have the option to choose items they wish to bring to your event.
Please note, the ability to edit items on your WTB list is not currently available.
Evite Tip- To have greater participation in your "WTB", include additional instructions on your invitation's "Message" section, or utilize the "Send a Message" functionality to further alert your guests of this option.