#1: Open your theme in the html/css editor
#2: Navigate through the templates to "Forums -> Topics -> Postcontainer".
#3: To reference the join date we use (this can be used for any member options in your core_members table, replace "joined" with the column you want to post (for raw data add |raw before the closing bracket))
#4: Pretty it up a bit with
#5: Add that wherever you want it to show, play around with it a bit