You can't have a mixed group chat between iMessage and SMS. If one person leaves the iMessage ecosystem there's no way to update the group chat to include you.

You can't have a mixed group chat between iMessage and SMS. If one person leaves the iMessage ecosystem there's no way to update the group chat to include you. This is like saying "I was in an IRC channel with friends but I left for Slack and now I don't get their messages" Stop whinging.