Hello 2lookingdown,
Not technically southern Ohio, but here are two organized clubs that are relatively close to Hillsboro:
Columbus Muzzleloading Gun Club:
https://sites.google.com/site/cmlgc1/ 
It's up near Columbus in Darbydale. I work in Columbus and live near Greenfield, so it's not that bad a drive (we have two guys who come to the shoots each month from Eastern Ohio, which is about a 2.5 hr. drive). I've been a member of this club since 1996 and we have a great bunch of shooters here. Next shoot is March 24.
Bill Moose Muzzleloading Gun Club:
https://sites.google.com/site/billmoosegunclub/home Many of the same guys who belong to the Columbus Club also belong to this club, so it's there's a great bunch of shooters here as well.
Next shoot is April 6 & 7.
There are other organized clubs in Cinci, Dayton, And Xenia, but have never been to their shoots. I went to a Groundhog Day Shoot at the Xenia club many years ago where they shoot at eggs dangling on strings at various distances for the prize of a frozen chicken. That was a BLAST! You can find info for the other Ohio Charter Clubs at this link:
http://nmlra.org/business-directory/wpbdm-category/ohio/ I know that the
Miami Rifle & Pistol Club:
http://www.miamirifle-pistol.org/muzzleloading.php has monthly shoots. I've never shot at this club, but once again we have a few shooters from here that come to the Columbus shoots each month too.
By the way, do you belong to the National Muzzle Loading Rifle Association? If not, please consider joining us as we need your membership and support.
Hope this helps.
galudwig
