All plans come with a predefined number of users. If you need to add more users than your current plan offers, you will be able to buy extra bundles of users. Each bundles includes 10 extra users, giving you the flexibility to grow as you like!
Unless you specifically change the billing period (eg: from monthly billing to yearly billing), you will only be charged on the day of your previous billing date. The amount will be calculated on a daily prorated-basis. If you want to learn more about how proration exactly works, refer to our payement gateway provider Stripe and their documentation about this topic here.
If you'd like to learn how to invite your team members, check out this article.