User Activity based Expiration Policy for Office 365 groups is now generally available!

Alex Ouretski , AndrewWarland the documentation says about retention and expiration:

If you have setup retention policy in Security and Compliance center for groups, expiration policy works seamlessly with retention policy. When a group expires, the group's conversations in mail box and files in the group site are retained in the retention container for the specific number of days defined in the retention policy. Users will not see the group, or its content, after expiration however.

Source: Office 365 Group Expiration Policy