Change default calendar AccessRights in Microsoft Office365's Exchange

Here are wo methods four your reference:

1.You can create new mailboxes with changing default calendar permission via a script. Please review to the answer of the following thread:

http://social.technet.microsoft.com/Forums/exchange/en-US/1229ed0f-1441-47a6-a6d2-14f6e0846892/default-calendar-change-to-shared

2.Navigate to Exchange Online admin center, then click on Sharing under Organization, you can change the Individual Sharing policy rule to meet your needs. Set up "Sharing with a specific domain" to match your Office365 domain, and then select the appropriate sharing option depending on what you want to do.