I have been running such a jam for over four years. Our jam is open to all skill levels. We use meetup.com as a platform to communicate. Check us out at http://www.meetup.com/NJ-Acoustic-Music-in-the-Park/
We actually have two flavors of jams to meet different desires.
In our Thursday night sessions every site in a large circle and all peform songs together as a group. All instruments are strictly acoustic except for an electric bass. We use a setlist of songs suggested by members ahead of time, so we have direction and members can familiarize themselves with the material ahead of time.
In our Wednesday night sessions we divide up into groups of 3 or 4 and prepare songs for performance in an open night later in the evening. The open mic allows electric instruments and a small PA.
Our group is very popular, successful and enjoyable. Good luck with getting your group off the ground.