Insert multiple rows into a table with only an IDENTITY column
As of writing this, there is no way to insert multiple rows into just an IDENTITY column using an INSERT statement. The DEFAULT VALUES placeholder stands for just one row. And the INSERT ... SELECT syntax has no extension to support the same functionality as the DEFAULT VALUES clause.

Instead, you can use a MERGE statement to achieve the goal:

MERGE INTO
  dbo.Groups AS tgt
USING
  dbo.MySource AS src  -- &lt;&lt; use your source row set here
ON
  1 = 0
WHEN NOT MATCHED THEN
  INSERT DEFAULT VALUES
OUTPUT
  INTO @output (ID)
;


The ON 1 = 0 clause basically turns the MERGE into a pure INSERT, because the explicitly false condition causes all the source rows not to be matched and thus trigger the WHEN NOT MATCHED THEN branch of the statement. Now, the WHEN NOT MATCHED THEN branch expects a single-row insert definition, where the familiar DEFAULT VALUES is perfectly valid. As the result, you effectively get an INSERT ... SELECT statement with the functionality of INSERT ... DEFAULT VALUES for an arbitrary number of rows.