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.
To help curb large fluctuations in your costs, each new member of your workspace is granted a 2-day grace period where their usage of Fellow will not be reflected in your prorated bill. We hope this gives workspace administrators a little time to remove users who were added/invited to the wrong workspace.