Most collaborative software pricing is designed to charge you per user regardless of how many people on your team are actively using the software. You usually have to front-load all the seats you expect to use in an entire year, or you need to pay for extra seats based on whatever tier your team fits into (e.g. 2-10 users, 11-20 users, etc).
At Rive, you only get billed for the users that are on your team. When you first create a team, you'll only be billed for one user. We'll only bill you for new users as you add them. If you remove a user, we'll give the prorated credit back to your account.