Will I get charged if I add an additional user?

Nope!

Subscriptions with Annature aren't tied to users/licenses, you're free to use Annature however is required with access for all team members included.
We generally suggest creating a new account for each team member as opposed to sharing a general account. This provides greater visibility over who is sending envelopes and ID verifications in the organisation, and easy to remove access if someone leaves the business.
Learn more about creating and editing team member accounts here.