or try to remember the thread you saw them on...did you post to it too..
then you can look on your profile, and search your posts and find the thread,
or do a search for the thread by title, or a peculiar word in the discussion.
Ditto rita,
OR... if you don't have the person on your friends list and can't find a recent post, you can use the "Search this Community" tool (directly above "Recent Activity") and search for the person's ID -- what will pop up is all their most recent posts, which you can then use to get to their profile page so you can leave a note. Hope this helps! :)
~eureka
If the person is one of your friends "meaning you have added them as a friend" then go to your friends and just find them there, otherwise the only way i know is if you see a post from click on there name, going through the whole forum could take awhile.
peace
rita