PLUS the Default Sub Setting's default should be that of the group's creator as the logical place to start from (not California's).
I like this idea, where the default account settings (time zone and display) are concerned.
prompt Groups.io accounts (email address) to set (or verify) their time zone before they join each group,
Each user has a single time zone setting (in their account), not one per group membership. I think expanding this to one per group would be too confusing for the users.
Maybe the user could be alerted to the fact that the group's Default Sub Setting for time zone differs from his/hers. A banner on their Subscription page perhaps, with a link to their Account Preferences?
But that wouldn't affect users that are Direct Added, or who accept an Invitation by email, or who +subscribe by email - until or unless they visit their Subscription page for that group.