@bob3bob3 is correct in everything he said. Thanks for helping out Bob.
Comments are definitely the most effective way to get in touch with someone who has contributed imagery. They will receive an email if they have notifications enabled and they will also see the comment in their feed in both the web and mobile app.
As Bob pointed out, signing up to the forum and to Mapillary are separate processes for the time being, so you might not find everyone from Mapillary on the forum.