What solutions are there for ISPs that mark a post as Spam causing Groups.io to remove the member until they reply to get back in again?

Don't mark group messages as spam, and routinely check your Spam folder for any group messages that may have been misdirected there. If you find any there promptly mark them as "not Spam". Do not delete them from the Spam folder, or allow them to be automatically deleted from there.

Also, if your email provider supports one or more of these actions:

• Create a filter to direct all group messages to your inbox
• Add groups.io and/or frequent poster's email addresses to your Address Book
• Add groups.io to a "safe senders" list.