Calendar in list but not in sidebar
I solved this by going to Preferences (⌘+,) -> Accounts -> Google Account -> Delegation (tab) -> Click on My Meetups.

I can now see the meetups in the calendar :)

Warning: This is a viable solution only, of course, if you don't use the delegation feature -> If you do, you will give (unneccesary?) access to your Meet ups to your delegates, which may not be what you want.