create/remove Global Address List

new-GlobalAddressList "Default Global Address List" -RecipientFilter {(Alias -ne $null -and (ObjectClass -eq 'user' -or ObjectClass -eq 'contact' -or ObjectClass -eq 'msExchSystemMailbox' -or ObjectClass -eq 'msExchDynamicDistributionList' -or ObjectClass -eq 'group' -or ObjectClass -eq 'publicFolder'))}

or create a GAL for the DAN company, run the following command.

New-GlobalAddressList -Name "DAN GAL" -IncludedRecipients MailboxUsers -ConditionalCompany dan

Remove-GlobalAddressList -Identity "DAN" -DomainController ex7k.dan.com

 or delete the gal using "adsiedit"

 for hidden a mailbox use this command:

Set-Mailbox "Mailbox Name" -HiddenFromAddressListsEnabled $true