Ok, this may be a question for Dave since the grouped object will not snap to the grid and there might a special thing to do first.
Another way to do it from your sketch:
1. Draw another horizontal line like the upper horizontal line and place it at the snap location you want the vertical lines to extend to.
2. Select all the vertical lines without grouping.
3. Use the Tools->Combine->Extend->End command
4. All lines should extend to the new horizontal line.
5. Delete the horizontal line if necessary.
You can also do the same thing one line at a time by just double-clicking on the end of the line and it will extend to the horizontal line.