In Microsoft Skype for Business Server 2015, you add a user to a distribution list. Then, the response group agents are synced from this distribution list. When the Skype for Business server doesn't find a response group agent, it will create the group agent. However, the server still can't find the agent.
This issue occurs because the users don't have the msRTCSIP-OriginatorSip populated so that the filter that's used to find the group agent isn't correct.