When the user sees the message "You are not subscribed to this group with this email"
I think it would be helpful to shorten the pink area up to the length of the text You are not subscribed to this group with this email
then immediately to the right of the message show the button "Join This Group"
I find it difficult to support this proposal. It's likely that the person is already subscribed. Encouraging them to subscribe a second address does not serve to explain what happened and will only cause further confusion when they begin to receive duplicate messages from the group.