An active user is any user that signs in at least once during your monthly (30 day) billing cycle. Once a user has logged in, they will not be counted again as an Active User during your 30-day billing cycle, regardless of how many times they log in.
Starter: 100 Active Users/Month
Advanced: 250 Active Users/Month
Premium: 500 Active Users/Month
Enterprise: 501+ Active Users/Month