You are letting waitlist members in before the public launch.
Hi {{first_name}},
Your {{ProductName}} access is ready: {{access_link}}
Start with {{first_action}}. It should take about {{time_estimate}}.
If anything breaks, reply with the rough steps that led there.
{{Sender name}} The invite is the event. Treats access as the gift, not the marketing copy around it. Single primary action.
A specific link. A one-line "what to do first" instruction. An offer to reply if anything is broken.
A multi-step welcome flow before they have logged in. Heavy positioning copy. Promotional tone.