Hey Mike,
Based on your description, I would assume you're on MeetingPlace 8.0 with MP owned user profiles.
The creation of user profiles in this deployment should only be done on MeetingPlace. The user profiles will not exist on WebEx until the first time that the end user logs onto the system using his/her credentials. If you seperately created the user profiles on the WebEx site as well as integrating from LDAP, you may encounter this sort of error. In this situation, erasing the user profile and letting it regenerate will likely resolve your issue.
If this doesnt work, you may want to check the status of your TSP connections to WebEx. This can be done by logging onto the CLI of the MP application server and entering the following command:
tspp.py -t
If your TSP connections show as disconnected, you will want to review the following document:
http://docwiki.cisco.com/wiki/Cisco_Unified_MeetingPlace_Release_8.0_--_System_Requirements_for_Cisco_Unified_MeetingPlace_Release_8.0#TCP.2FUDP_Ports_Used_in_Cisco_Unified_MeetingPlace_Scheduling_Deployment_Options
Hope this helps,