Given the unsophistication of many members (which means that they have no idea of spam filters, let alone what to about them), and the difficulties of understanding what anyone thinks is going on (are e-mails sent to spam 'accidental' or 'deliberate' (and if so to what end)?), anything intended to cause group members to take any particular action is unlikely to succeed, certainly beyond a simple click (and frequently even that).
My thought is that unsubscribing people, and allowing them to rejoin the group, is perhaps a bit brutal, and will frequently lead them to yo-yo out and back into the group. and not necessarily resolve the 'marked as spam' problem.
Better to set them to 'no mail', with a message saying something like
"Messages sent to you have been reported as spam, so we (groups.io) [assume you do not wish to receive them and] have set you to 'no mail' and won't send any more. If you no longer wish to be a group member then <do this>. Alternatively you may continue to read messages at <web address>. Should you wish to receive e-mails please ensure that they are NOT marked as spam (this may be done automatically by your e-mail provider, so you may need to change your settings with them), and change your e-mail delivery preference at <webpage> " ?
There is the question of what to do about groups where 'no mail' is not allowed - either it should be forced (overriding group owner's setting) or (in this case only) that they should be unsubscribed.
Jeremy?
