I'd post a modified version of the groupmembers for this mod, but since I don't have it downloaded I'll just post the code I added to my own group window:
(Added to GroupMembers/GroupMember1/MemberInfoPage)
Code:
<Button LocalTooltip="Disband" Location="8,4" Name="Disband" OnPress="leavegroup leavegroup" ScrollExtent="10,10" Size="10,10" Style="/ButtonStyles.close_button" Tooltip="Disband"></Button>