I’ve Just Accidentally Sent A Message To The Entire Group.
Can I Delete It?
Short answer: no.
Unfortunately there is no way to delete your message after you send it. Once you hit send, your message goes to group.io's servers and from there is forwarded to the email servers of everyone in the group. That’s hundreds of different computers, run by many different companies (or people), and there is no way to remove the message from all of those machines.
You can delete the message from the groups.io archive. Login to groups.io and find the message. If you posted it you will have an option to delete it. If you have trouble with that you can ask the moderators – via firstname.lastname@example.org – to delete the message from the archive for you. But that is all the moderators – or anyone – can do. Once the message is sent, it’s forwarded to everyone’s inbox and it can’t be deleted from there.
So please be careful about where your messages are going. Check the TO line carefully before you click “Send”. See the question How Do I Send Messages To The Group? for some best practices and suggestions.