OneDrive groups is feature where users can create a groups and share files with multiple users in that group. Since people can achieve the same by just sharing public link now, the groups feature was retired but the users who were using groups already were still able to access it.
Microsoft has now sent a mail to the OneDrive group users informing that the OneDrive group feature will be removed completely and existing groups will not be accessible from 16th October 2015. Check out this screenshot of the email that was sent by Microsoft for more information:
These are the help links provided in the mail to move your files and then share them again: