You have two main options for publishing on Medium as a company, brand, or organization: publish from a regular user account under your organization's name, or set up a publication and have multiple people in your organization write as themselves and publish in that publication. Or you can do both.
Each user account has a profile page that is visible to everyone and can be used as your public persona on Medium. Most brands create a general user account for their organization in the same way they do on other social media sites. User accounts are great for publishing things that the organization as a whole (rather than a specific individual) has to say—corporate communications, announcements, and the like.
Publications allow you to grow your brand’s presence on Medium beyond a single account. Publications are essentially content hubs that allow stories published by many authors to live in one place. Each publication is owned by a specific user account, but can publish stories submitted by multiple accounts. As the publication's owner, you'll be able to customize the layout of its homepage and add subpages (tabs) for specialized content. The publication will live under medium.com/your-publication-name.
To learn more about publications and how to create one, see this article.