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Expands current group.
Shows the rows or columns in the current outline group again after the group has been collapsed. If the group has a collapsed parent group, this method clears the current group's collapsed state, but the grouped rows or columns remain hidden until the parent group is expanded.
void Expand()
Sub Expand()
worksheet.Range["2:5"].Group();
IGroupInfo group = worksheet.Outline.RowGroupInfo[0];
group.Collapse();
group.Expand();