Use the HeaderHeight property to change the height of the group's header bar. Use the HeaderVisible property to show the group's header bar. If the HeaderSingleLine property is False, the HeaderHeight property specifies the maximum height of the control's header when the user resizes the columns.

For instance, the following sample displays the group's header bar using multiple lines:

With ExplorerTree1.Groups.Add("Group 2")
    .BeginUpdate
        .HeaderVisible = True
        .HeaderHeight = 32
        With .Columns(0)
            .HTMLCaption = "Line1<br>Line2"
            .Width = 128
        End With
        With .Columns.Add("Column 1")
            .HTMLCaption = "Line1<br>Line2"
            .Width = 128
        End With
    .EndUpdate
End With