Of mine, Rhys will have the most to do with him! He's his yearmate (Hufflepuff 02) and a fellow former Auror who was forced to retire last year after he was injured in fight with Rodolphus. As an Auror, he was very by-the-books and friendly with everyone, so he'd probably be friendly with John too! He currently runs a B&B with his lovely wife Nora where he is definitely not secretly sheltering Muggleborns. :)
I also have Dedalus, who is an OG Order member/illusionist, and Keats, who is a nerdy artist/DE, who you may run into while they break the law or whatever.