Hi Steven--
That's handled by the Confirmation Request feature. There's a video that I made for the previous version of WheresTheGig, but the process is pretty much the same for the current version. The Confirmation Request review page is at "Services" => "Confirmation Requests". I recently added a feature so they can be sent via email, SMS, or both. Please get in touch if you want to know more details about how it works after watching the video.
There is also a page where your members can enter their unavailability dates up front. This is at "Blackouts" => "Set Blackouts". You can receive an email notification when members update their availability if you enable the option on the Settings page, and any availability conflicts will be shown on your gig list and calendar.
--Ethan