Kelsey, it's unfortunate you couldn't have arranged open-jaw tickets for the small group that will be heading to Venice and Rome. An outbound flight from Rome would have been much easier than returning to Munich. The fact that you only have 4-5 days for this part of the trip makes it especially challenging.
The only way I can see for this to work would be to drop Venice and focus on Rome. Take a budget flight from Zurich to Rome (try Air Berlin as they have flights to Rome/FCO from Zurich). Your cost for the flights will depend on what day you're travelling. Note the usual restrictions with these carriers - restrictive baggage limits and tickets that are usually non-refundable and non-changeable.
On the return trip, use a budget flight from Rome to Munich (again, Air Berlin. The cost of the flights will add somewhat to your travel budget, and of course there may be a concern in getting seats for all of your group.
Another option would be to drop Rome and focus on Venice. Travel by train from Zurich to Venice is about 7 hours, minimum one change, reservations compulsory. The trip from Venice to Munich is also about 7 hours, again minimum one change and reservations compulsory. Even travel by train between Zurich-Venice-Munich is going to use pretty much two days of the 4-5 days you have for that part of the trip.
Hope this helps? Good luck!