You can add as many users to your organization as your plan allows. By default, ProtonMail Professional plans can use up to 10 email addresses (including custom addresses). Each user added to your plan (after the first) adds five email addresses to your account.
So, if you add one user to your account, you will have 15 email addresses, if you add two users, you will have 20 email addresses, etc. Once addresses have been added to an account, an organization administrator can assign them to users in any way desired.
For example, an account with just two members (the account owner/admin and one added user) can use up to 15 addresses. The admin decides to assign eight addresses to herself, three to the user, and keeps four in reserve for use later.
Users can also be removed from your Organization.
How to add and remove users and addresses to your account
1. Log in to mail.protonmail.com and go to Settings → Go to Settings → Account → Dashboard → Your plan → Customize subscription.
2. Under ProtonMail customization, add users to your account using the + icon (or remove them using the – icon).
Every time you add a new user, five addresses will be added automatically to your account (or removed if you remove users).