Actually it should be fairly simple, copy the default
eq2ui_mainhud_groupmembers.xml into your custom UI folder, open it in a text editor (I tend to recommend notepad++) and exchange just the text "GroupMember1" with "GroupMember5" and "GroupMember2" with "GroupMember4" (exchange it both ways, ie 1 becomes 5 and 5 becomes 1, same for 2 and 4).
Edit: Forgot something, it's not quite as simple after GU44, you also need to exchange Group_1/Group_5 and Group_2/Group_4. Each of those should be appearing 5 times.