Jonathan,
Does this mean that when I mark a message as spam on my email
program (Thunderbird) it automatically sends a message back to the
sender with the information that I marked the message as spam, and if
that sender happens to be any Groups.io group I will be automatically
removed from that group?
Maybe.
That will depend on your email service, the list service, and on how you've configured Thunderbird.
Some webmail services (including big ones like gmail) have an arrangement where email list services (such as Groups.io) can sign up to be informed when a mail user marks a message from the list as spam. Groups.io is signed up with many email services because with some of them that's the best way to help ensure that other list members receive list messages. Otherwise the marking of a list message as "spam" goes into the mix with whatever other factors that email service uses to filter spam, adversely impacting other list members who use that email service.
I could understand this if someone marked it as spam on the web site,
but not with the email program on my home computer.
Indeed, the other factor is whether your email service knows anything about what you've done in Thunderbird.
If you are using POP3 to retrieve messages then I think the answer is a flat "no". But if you are using IMAP that raises to a "maybe", because Thunderbird may move messages between server folders, including from your Inbox to your (server side) Spam folder. You'd have to look and see how you have Spam handling configured in Thunderbird to know whether marking a message causes an action at your email service.
Should I be warning the members of my groups that this could happen
inadvertently?
As a general statement, yes.
There have been reports that some email services will even send a report (and trigger an unsubscription) without any explicit action on the recipient's part. If the message is automatically filtered to the Spam folder, and the user doesn't remove it from there, then eventually the Spam folder gets emptied and a report is triggered. That's nuts, IMO, and I hope that's not still the case, but when Groups.io first signed up for receiving reports there were cases which could only be understood as having happened automatically.
Shal
Shal Farley