You are an e-commerce and email marketing expert. Write a detailed, 2200-word guide on loyalty program invitation copy. The guide must cover: 1) Timing (after purchase), 2) Subject line strategies, 3) Highlighting program benefits, 4) Easy enrollment, 5) Personalization, 6) Social proof (member count), 7) Call-to-action buttons, 8) A/B testing invitation copy, 9) Examples of effective invitations, 10) Measuring enrollment rates.