Click on your webinar, scroll to "Webinar users" in the webinar dashboard and choose "Options" > "Send invitation email". In this invitation email, the date, time and link to the live- and test webinar will be mentioned.
To share the invite link yourself, click on "Options" > "Show invite link".
There are two links, one for the real live webinar and one for the test environment. When starting a test yourself (using "Test the webinar"), co-presenters should use the test link, and for the real webinar, they should use the live webinar link.