You can define the group names per project in MMX Properties - Project options:
A single identifier is treated as a group prefix which has to be followed by a dot (like System, VCL, FireDAC).
You can also use wildcards like Rz* for all units from Raize Components (KSVC).
Unit names not covered in one of the schemes above can be listet in brackets.
This is the setting for MMX itself:
(ToolsApi,DesignIntf,DCCStrs,DockForm,TabDock);Winapi;System.Win;System;Xml;Vcl;VirtualTrees*;Rz*;Tb*;Png*;MMX
The settings are stored in the dproj file.