If a simple signup has reached capacity, you can give people the option to reserve spots with a waitlist in the case they become available.
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Waitlists in Detailed Signups
Waitlists don't exist in detailed signups. However, if you want to have a waitlist available for your detailed signup, follow these steps.
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Create a hidden selection that is a duplicate of the original selection.
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When the original selection becomes full, set the hidden one to show on Church Center.
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If spots open up for the original selection, change the selection on the registration and send them a balance due email to let them know they've been moved to active.
From the Selections tab in the Manage tab, check the box to enable the Waitlist for selections.

As people register for the signup, the Registrations number will increase. If someone has already selected the waitlist, the box will be grayed out.
As people sign up, they will be added to the Waitlist tab. If spots become available for their chosen selection(s), you can promote them from the Waitlist to refill those spots. Click a registration on the Waitlist tab to promote it.

You'll be taken to their registration where you can click Promote to move the registration from waitlist to active.

When you promote a registration, the registration contact will receive an email about their registration.
