Cisco CallManager 4.2.3 and later allow anyone in an ad-hoc conference to add participants. But, this feature only works when the Advanced Ad Hoc Conference feature is modified.
In order to fix this issue, complete these steps:
1. Open the Cisco CallManager Administration page.
2. Choose Service > Service Parameters > CallManager.
3. Scroll to Advanced Ad Hoc Conference Enabled, and change it to True.
Refer to this article for more information on conference considerations:
What are the considerations to follow in order to configure the Meet-Me Confrence in Cisco CallManager 3.x and 4.x