After you have added a Custom domain to your account, you will have the ability to create an Organization. This will allow you to have multiple users using your custom domain for their email addresses.
To create an Organization, visit the Addresses/Users tab of the Settings of your account.
Select Enable Multi-User Support to create your first Organization. When you do so, you will be prompted to pick a name for your Organization. This name for private internal use only, it will not be seen outside of your account.
Upon picking your Organization’s name, you will be provided with an opportunity to select your Organization’s key strength. This key will be used to encrypt all data in your user’s accounts (exception with private accounts), providing you access to the data while preventing ProtonMail from accessing this data.
Your organization password is what protects your key from ProtonMail. It is critical that you do not loose or forget this password as it is the way you will gain access to your organization in the future. Lose of this password will result in the organization being un-editable.
Now that your organization password and keys have been set, it is time to allocate storage space to your admin’s account. The storage space available to your organization is determined by the level of storage you have selected in your Protonmail plan. You can increase it in the future if you need to. You will have the ability to select the storage amount each user has access to. At this step, you will be deciding how much space to allocate to the administrators account. This storage will be used for the administrator’s email contents, email is the only use of this storage at this time.
You should now see an Organization appearing at the top of your Addresses/User tab in your Settings.
Once you have verified you can setup the Users of your Organization.
Below are some additional articles to assist you while setting up ProtonMail Business accounts:
Step 1: Setting up Your Domain(s)
[You are currently here] Step 2: Creating your Organization
Step 3: Setting up User(s)
Step 4: Migration of existing email messages (Coming 2018)