>>show the detail records of that individual group
I am not sure I understand.
If g is that individual group, calling g.ExpandAllGroups will expand all groups under that group to display all the records under that group. You said this was not what you wanted, but how do you want to see all the records under this individual group without expanding all the groups between the first group and the records?
For example, if I add a GroupExpanded event handler as below, clicking on any group will expand all inner groups under it.
private void gridGroupingControl1_GroupExpanded(object sender, GroupEventArgs e)
{
e.Group.ExpandAllGroups();
}