After long click certain group, click 'Set Group SMStone'.
Then, ringtone lists will be shown up.
Select ringtone and click 'Ok' button.
Menu > Setting
Check 'Enable group SMStone'.
You need to check ‘Default SMS notification sound’ and select specific ringtone for receive SMS/MMS from contacts which do not have any information or register to any group.
You need to make clear other SMS alarming system. Otherwise, it can be duplicated when receive SMS.
For Nexus One smart phone, here are steps to clear basick SMS alarming sound.
1. Operate SMS applications
2. Go to Menu > Settings > Select Ringtone
3. Check 'Silent'