It's currently not possible to export a machine group directly to any text file, but you could use the steps below to get the machines exported to CSV:
1) Go into your machine group, then click on 'View in Machine View'.
2) Once it transfers you to machine view, you can expand the grouping to see all the machines listed.
3) Select all the machines (Select +A), then right click anywhere in the highlighted area, and choose 'Export selected machines to CSV'
The CSV export should be usable as a text file or you can manipulate the information when opening as a spreadsheet.
I hope that helps. If you want a specific feature that we don't currenlty have I would also suggest that you submit a feature request at Shavlik Feature Request Form.