Subscribing to your Google, Apple, Outlook, or other Calendar as a Public Feed can make your life easier since you won't have to deal with passwords and other authentication methods. The only disadvantage is that your events will be read-only.

Go to Strategy Settings → Calendar Sync → Add Calendar → Public feed and then paste the link you find below.

Go to Settings at the top right at https://calendar.google.com/:

Find the Calendar in the left sidebar that you want to subscribe to in Marvin under "Settings for my calendars":

Scroll down until you find Secret address in iCal format, and copy and paste this link into Marvin:

Go to https://outlook.live.com/mail/0/options/calendar/SharedCalendars and choose your Calendar from the dropdown under Publish a Calendar. Then choose "Can view all details" and click "Publish".

Click the link next to ICS to copy it to your clipboard, then paste this link into Marvin:

Open Calendar.app and find your Calendar in the left sidebar, probably under iCloud:

Right click and choose "Share Calendar..."

Click the checkbox that says Public Calendar so that it's checked, then click Done.

Then right click "Share Calendar..." again and this time you will see the link that you can copy and paste into Marvin:

Other Calendar and services (like Meetup) provide access to Calendars using public feeds. Look for words like "publish" and URLs that start with "webcal://" or end with ".ics".