"Straight off" seems like a strange choice. When someone plans to get married, they don't immediately invite a select few people and then start inviting others later. They generally sit down and write all their invitations at the same time. I would say something like "Don't worry. If I ever get married, you'll be the first to know". If you're simply trying to reassure the person that they will receive an invitation to the big day, then "Don't worry. If I ever get married, you'll be invited".
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