Domain based email ids
If you have a large user base you can even import users in bulk. Create a new sheet and export it as a .csv file. The .csv file can be created in Zoho Sheet. Give the header as 'Username', 'Password', 'First Name' and 'Last Name'. Fill in the required details and export it as a .csv. After clicking the Browse window and selecting the file, all the users will be imported into your organization.
Please communicate the password to the users so that they can use the credentials to login.