Add-MailboxPermission -Identity "MAILBOX" -user "Mailbox Alias or Mailboxgroup Alias" -AccessRights FullAccess -InheritanceType All
Categories:
Exchange,
PowerShell
Add-MailboxPermission -Identity "MAILBOX" -user "Mailbox Alias or Mailboxgroup Alias" -AccessRights FullAccess -InheritanceType All