Facebook users are reporting a rather serious problem which sees them re-added to groups they left even years ago. While it's an inconvenience for users, who have to unsubscribe again, sometimes from hundreds of groups, it's an even bigger problem for the members of the groups who believed they were sharing things privately.
The issue could be a privacy concern for many people, as sensitive information may have arrived in the hands of people it was not supposed to. There's a large number of scenarios where this is undesirable.
Facebook has confirmed the problem and is actively working on fixing it. Meanwhile, users that are finding themselves members of groups they are no longer interested in have to deal with the problem by unsubscribing from them once more one by one.
As for group admins or members, the only option is to boot out the people that have suddenly made their way back into the groups, but it may already be too late in many cases – the info may have already been compromised.