At the start of each billing cycle (monthly or yearly), your workspace is billed for the number of active users at that time. Think of it as pre-paying for a set number of your teammates to use Fellow. If you deactivate a user, or they leave your team/workspace, you can replace them with another user at no extra cost.
However, if the active user count increases during a billing cycle, i.e. a new teammate joins your team, your next monthly bill will be prorated to include their active period. Active users are calculated daily and billed monthly. Note that this proration also affects yearly plans, and therefore you would expect a monthly charge.
Each month, on the date of your billing cycle we apply proration adjustments. (ex. subscription started March 15th, subsequent bills will be on the 15th). If there is an outstanding balance due to proration adjustments, you will receive a bill with charges or credits for the outstanding prorations.
For monthly plans, this proration adjustments will be added onto your normal bill. For annual plans, you might receive a bill of proration adjustments only.