Some members on LinkedIn set restrictions that require you to have their email already in order to send them an invitation.
With this feature enabled, Growth-X will discover the email and you will be able to send the invite. You'll also be able to export such emails from the Prospects tab.
How to activate the feature:
Go to the 'Campaign' tab in the app, create a new campaign or select a previous one.
Scroll to the bottom of the campaign template and toggle the option 'Invite people who require email using data credits'
Hit 'require email using data credits' and save.