Per default the calendars of shared mailboxes in Office 365 have “FreeBusyTimeOnly” permissions applied.
You could create a new sharing policy and change the applied policy on those accounts.

I however chose to just change the sharing permissions on the specific shared calendar.

Firstly you need to connect to the Office 365 service though PowerShell.
Secondly you need to figure out which permissions you’d like to apply to the calendar.

None
Owner
PublishingEditor
Editor
PublishingAuthor
Author
NoneditingAuthor
Reviewer
Contributor
FreeBusyTimeOnly
FreeBusyTimeAndSubjectAndLocation

So to change the Default permissions on MYUSER to LimitedDetails you would have to use:

Set-MailboxFolderPermission MYUSER:\Calendar -User Default -AccessRights LimitedDetails

If you would like to change all permissions to LimitedDetails you could use:

Get-mailbox | ForEach-Object { Set-MailboxFolderPermission -Identity $_":\Calendar" -User Default -AccessRights LimitedDetails }

You can find more information on this subject at TechNet.

Hope this helped you. Let us know if you’re having trouble, and feel free to share the information.