It's time to start looking into other collaboration tools if you've been using OneDrive Groups. Microsoft plans to close this feature on October 16th. So all the files shared with you won't be available to you past that date if you don't save it to your personal OneDrive account. It would seem Microsoft has started to limit users' access to the feature as you can't create new Groups or add people to your Groups. You can only work with what you have now. 

Source: PCWorld