If you are an IT person looking to allow employees to integrate their own calendars (work or personal), here are the steps to do so:
1. Go to admin.google.com
2. Find the Workspace Marketplace Settings on the left navigation
3. If you are blocking users from installing apps, select the "Allow users to install only allowed applications" option. Manage the allowlist and add Together as an application.
For information on how a user can integrate their individual calendars, see this article.