Notifications in Groups

There's no way of forcing a new default in the UI at present. Instead of creating the group via the UI, you can create it using PowerShell and ensure that all members are subscribed that way by setting the AutoSubscribeNewMembers property to $True (New-Unified Group -AutoSubscribeNewMembers $True...). 

Alternatively, you could scan for all groups (on a regular basis) where AutoSubscribeNewMembers is set to $False and set the property to $True

[PS] C:\> Get-UnifiedGroup | ? {$_.AutoSubscribedNewMembers -eq $False } | Set-UnifiedGroup -AutoSubscribeNewMembers $True

As you might have some groups where you don't want autosubscription, a more sophisticated solution would check that a group really wants to have autosubscribe set to True before forcing the matter.

For more details on how to do all of this, see TechNet or read chapter 9 of Office 365 for IT Pros...