No, you can't add more than 10 users. The free plan allows you to add only upto 10 users to your account, beyond that you need upgrade to a paid plan.
When you upgrade to the paid plan, you pay for all users and not just for the 11th user.
Starter - 10 users * 0 USD/user = FREE
Pro - 11 users * 5 USD/user = 55 USD per month (when paid annually - 6 USD/user when paid monthly)