Marvin has beta calendar sync with Outlook and Exchange. Note that for now (as of Aug 6, 2020) it doesn't work in the desktop app and works best in Chrome, Firefox, or the new Edge (based on Chromium). There is also for now limited support with recurring events (changes to recurring events you make in outlook or Marvin are not synced with each other).
Follow this guide if you'd like to setup your own Azure AD app registration. This lets you configure exactly which permissions you want Marvin to have.
Check out this FAQ and troubleshooting guide if you have questions or issues.
And don't hesitate to contact support from within Marvin if you have other questions.