If you need to enable Office 365 Archive for all users:

Get-Mailbox -Filter {ArchiveStatus -Eq “None” -AND RecipientTypeDetails -eq “UserMailbox”} | Enable-Mailbox -Archive

If you need to enable Office 365 Litigation hold for all users:

Get-Mailbox | Set-Mailbox -LitigationHoldEnabled $true -LitigationHoldDuration  2555

Remove  “ -LitigationHoldDuration  2555″ to retain forever or change number for days to retain.