Five years ago, Michael McDerment built an invoicing system for his web design business. Today, FreshBooks lets nearly 400,000 users invoice their clients, track time and manage expenses electronically. Not all are paying — the service is free for users with up to three clients — but it’s growing fast. “We’re seeing double-digit growth for both users and paying customers each month since our May 2004 launch,” he said. McDerment, who is also a co-organizer ...