If you are trying to add a management group to an agent and get the following error:-
(Agent Error) The management group configuration could not be saved
Then this could be an indication that the management group info you are trying to add has been attempted previously and has been unsuccessful.
All you need to do is delete the following registry keys on the computer you are trying to monitor for the managment group you are trying to add and restart the health service:-
HKLM\Software\Microsoft\Microsoft Operations Manager\3.0\Agent
Management Groups\[MG name]
Parameters\Management Groups\[MG name]
If this does not rectify the problem then try a reinstall of the agent.