Hi, wouldn't it be best to disable posting and subscription to the old google groups and add an auto-reply message which indicates that they should subscribe to the new group?
Here are a few indications on how that can be done: auto-reply: https://support.google.com/groups/answer/9792589?hl=en&ref_topic=2458761 stop posts: https://support.google.com/groups/answer/2646833?hl=en There should also be a setting to disallow subscriptions, but I can't find that. Best, Matias
