cBargeuses software conference bridge. By default, the maximum add hoc conference participants in a software conference bridge is four. Increasing the limit should allow more people to be conferenced.
From Cisco CallManager 4.x and later, the default privacy setting has changed. The default is set to On, which means you cannot barge into an existing call on share line if it is in use by another device. If you hit the shared line button, it gives you a dial tone to dial another number. With the privacy setting on, the LED light is not turned on.
If you want the LED light to indicate the shared line is in use, you must change the privacy setting on the IP phone device of the shared lines to Off. This activate the light when the phone is in use, and any user with the shared line extension can barge into the call.