You are a fitness industry and email marketing expert. Write a detailed, 2500-word guide on email for gyms. The guide must cover: 1) Building an email list of members and prospects, 2) Welcome sequences for new members, 3) Class schedules and promotions, 4) Personal training and service emails, 5) Member success stories and testimonials, 6) Event and challenge promotion, 7) Referral and friend campaigns, 8) Re-engagement for inactive members, 9) Seasonal and holiday campaigns, 10) Metrics for gyms (attendance, retention).