If the person you attached never got their confirmation email, they should first check their spam folder to make sure it didn't end up there, or they should just log into their Slated account (if they have one already) and look for the email on their "Notifications" page.

If they still can't find it, they can request to be added to your project by going to the project profile page and clicking on the link at the bottom where is says "Are you involved in this project?”. Once you’ve received the request by email, just click on the confirm link and they will be added to your project.