Try setting the Group.IsExpanded property. Here is code that expands one level of groups from a button handler.
private void button1_Click(object sender, System.EventArgs e)
{
ExpandOneLevelGroups(this.gridGroupingControl1.Table.TopLevelGroup);
}
private void ExpandOneLevelGroups(ChildTable parent)
{
foreach(Group g in parent.Groups)
{
g.IsExpanded = true;
}
}