Hello from the Office 365 Groups Engineering Team

Short term, we're releasing group activity reports in the Report center so you can do some self-directed pruning of obviously inactive, unloved groups. Expect this in the next month. 

Later this year, we'll release time-based expiry where owners can reattest the use of their group to extend expiry. We won't release expiry until we release deletion recovery - so you can recover a group that's been undesirably expired.