^ I had the 8900 for a year and had no idea you could assign a ringtone for groups. Anyways, if you're still having trouble, here's how you do it step by step.
1. Contacts
2. Select the contact that you want to have a particual ringtone
3. Press OK
4. Press the down arrow key 9 times, or until you get to the Ringer section
5. Press the left soft key for Menu
6. Change
7. If it's a ringtone you've downloaded or sent to yourself, press the Right arrow key till you get to the Downloads
8. Select the ringtone and press OK to select
9. Press OK again to save