To create a Sublist follow these steps:
From the contacts tab click the contact list you would like to create a Sublist from.
Select the contacts to create the Sublist and click 'Add Tags'.
Add the tag you would like to tag the selected contacts. Use a tag you haven't used before so you don't get confused. Click 'Save'.
Click the 'Tags' button and all your saved tags will appear.
Select the tag you saved your contacts as for your Sublist. Click 'Search'.
The tagged contacts should appear. Click the 'Save as Sublist' button.
Your new Sublist will appear under the contact list you created your Sublist from.