Query and change calendar permissions of a single user

You have to be connected to exchange online by powershell.

Query calendar permission of a user

Get-MailboxFolderPermission joel:\calendar

FolderName           User                 AccessRights                                                                                                       SharingPermissionFlags
----------           ----                 ------------                                                                                                       ----------------------
Kalender             Standard             {ReadItems, CreateItems, EditOwnedItems, FolderVisible}
Kalender             Anonym               {None}

The folder name “calendar” depends on the user language settings. For example if the user uses german, the command has to look like this.

Get-MailboxFolderPermission joel:\kalender

Set a built in access role

Set-MailboxFolderPermission -Identity joel:\calendar -User Default -AccessRights Author

Set customized rights

Set-MailboxFolderPermission -Identity joel:\calendar -User Default -AccessRights ReadItems, CreateItems, FolderVisible

Source

https://docs.microsoft.com/en-us/powershell/module/exchange/mailboxes/Add-MailboxFolderPermission?redirectedfrom=MSDN&view=exchange-ps