BuddyPress.org

I’d use the approach @boonebgorges suggests – a custom table with these fields:
-user_id unique
-long
-lat

Then you don’t need any joins.

>individual forum posts are geo-tagged

Based on where they were created or based on the existing geo-tag for the user?
If the former, you can add a column to the custom table.
If the latter, you’re good to go.