Here's a way to manually get a closer like the opener (seemingly does not work on Windows):
attachGroupCloser =
Block[{$Context = "FE`"},
FrontEndResource["FEExpressions", "cellGroupOpenerInline"]] /.
{
Open -> Closed,
"CellGroupOpened" -> "CellGroupClosed"
};
FrontEndExecute@attachGroupCloser[NextCell[]];
There are surely ways to get this to bind at the stylesheet level, but it doesn't even matter as this is far from ready for primetime:

(note the excess whitespace)
Here's an explicit example:
CellPrint@
Cell[
CellGroupData[
{
Cell["Close me", "Subsubsection"],
Cell["goodbye cruel notebook", "Text"],
Cell["I'll disappear too!", "Text"]
}
]
];
FrontEndExecute@attachGroupCloser[NextCell[]];