Grant Exchange Calendar Rights

To view Calendar access rights:
Get-MailboxFolderPermission -identity "ManagingUser:\Calendar"

You must remove any user listed as Reviewer before you can add Editor rights.

Remove-MailboxFolderPermission -identity “ManagingUser:\Calendar” -user “PersonalAssitant”

Once Removed Add Editor Access rights:

Add-MailboxFolderPermission -identity “ManagingUser:\Calendar” -user “PersonalAssitant” -AccessRights Editor

Grant Mailbox Permission to single user

 There are many situations in which permissions to another user’s mailbox should be granted on Exchange Server. Sometimes it is for monitoring purposes, sometimes in order to send emails on behalf of someone else. Full access permissions give the highest level of access to a mailbox, and are necessary, for example, during a migration process. If you want to learn how to set full access permissions on Exchange 2007, Exchange 2010, Exchange 2013, Exchange 2016 or Exchange 2019



Get-Mailbox -ResultSize unlimited -Filter {(RecipientTypeDetails -eq 'UserMailbox') -and (Alias -ne 'User')} | Add-MailboxPermission -User User@Example.com -AccessRights FullAccess -InheritanceType all -AutoMapping:$false

Full Credit to:

https://www.codetwo.com/admins-blog/how-to-grant-full-access-permissions-to-all-users-mailboxes-exchange/