Hey Rachel, (on the forum) click on the users avatar/picture so it brings up their profile. Upper right if that screen is a green button “Message”
If you are using the imagery web page and are viewing someones image it is possible to comment on it. (Hover over the icons to find “Comment”.
It seems though that the forum and imagery pages are separate user systems. Not all photo contributors are on the forum and the user names can be different. Many however are the same.
(Sorry for busting in Ed)