Once a schedule has been published, click the "Invite" button when viewing a specific opportunity in the Opportunities pane to invite volunteers to sign up.
From here, you can either (1) select "Copy link" to share the link yourself, or (2) select "Compose email" to use our built-in emailer to send this schedule to your existing volunteers.
(Note that if a schedule is "Protected", only people who are already in your database as volunteers will be able to access that schedule.)