Collaboration Suggestion

Here’s the situation: Four people sharing an Excel spreadsheet (Excel 97, Shared Workbook) and updating constantly. Column A has times at 15 minute intervals. Row 1 has names. The users enter an address at an intersection indicating where a certain person is supposed to be at a certain time.
The users are experiencing some instability and there are some Excel features that are unavailable when workbooks are shared. That’s not a big surprise, because sharing workbooks stinks. I usually recommend against it before I even know what the issues are.
I recommended Google spreadsheets. I’m sure there’s a better tool, but I just can’t think of it. What would you suggest?