You can invite other users to your workspace by going to Settings, then Users where you will see a list of all active users in your workspace. Click Invite users.

You can invite multiple users to your workspace or individually invite them. You must enter the users' first name, last name, email and security role.

Only practitioners and practitioner admins are billable users. You'll be billed for these practitioners according to your billing cycle. Admin users don't count as billable users, so you can add them without being billed.