This idea is one that takes from others that have already been implemented in Instant Messengers almost universally. It's a simple concept of being able to customize your friends list and the order in which friends show via creating custom groups within the list itself.
(Important note: All the 'groups' referenced within this suggestion are referring to the Friend Grouping described below, and not the current Group system which is something else entirely.)
This is how it would be structured within your buddy list.
Friends >>
Person 1
Person 2
Person 3
Business >>
Person 1
Person 2
Person 3
Other >>
Person 1
Person 2
Person 3
Additionally, you would have the option to edit this friends list via an additional -new- tool/interface. The reason for needing the friend list to be edited outside of the friends list is simply because it would be too easy to drag peoples names around and mess things up if it was all done through one window.
So there would be a new additonal button added to the friends list. So the buttons on the right would look like this.
IM/Call
Profile
Teleport...
Pay...
Remove...
Add...
Edit
In the editor, the friends list would be displayed with a full list of your current friends, in the current layout you have setup in a view pane on the left side. On the right side you would have buttons like Add Group, Sort (alphabetically, first push is A-Z, second is Z-A), Remove Group, Duplicate Friend (This would allow for you to list a friend in multiple groups)
Additional options: The ability to expand and collapse individual groups, conference call a group (Ctrl+left clicking would allow to select more than one group).
A separate pop up window that allows for advanced options of group management. (Much like properties works for items)
-The ability to send inventory to the group via friends list
-The ability to block conference calls from users in specified group
-The ability to disable/enable voice calls from the specified group
-Customizable text colors and font options for the group names
-Sorting options, A-Z, Z-A, Custom
-Display offline members of this group (on/off)
-Custom name color/Text color for users within the specified group (would display in individual IMs)
-Custom IM Alert blink color for users of a specified group