Your just talking about the Conference bridge feature that comes with CallManager. Really all you need to do is have an IOS conference bridge set up or a hardward-based conference bridge with a corresponding router with the DSP farm config for enough conference resources, as the CallManager bridge can be rather limiting. To allow public access, If you have a PRI, you can set up one of your hunt numbers that is a dialable number and do a route pattern into the Meet-Me numbers.
Example:
One of the numbers in your PRI hunt is 2175551212 and the Telco is passing you 1212, just create a route pattern 1212 and put it in the same partition as your Meet-Me number and it becomes a publicly accessible conference bridge for outside users.
