Once you have added a co-presenter, you can send the invitation. When the co-presenter lost the invitation you can send the invitation again or send the links yourself.
Click on your webinar, scroll to "Webinar users" and click on "Options"> "Send invitation email".
This invitation email will include the date, time and link for the live and test webinar.
Share links yourself
Click on your webinar, scroll to "Webinar users" and click on "Options"> "Show invitation links".
There are two links, one for the real live webinar and one for testing. If you start a test yourself (via "Test the webinar") then the co-presenters must use the test link. For the real webinar, the co-presenters must use the live webinar link.