My adult family of 8 needs train tickets for three legs of our journey: Cologne to Rothenburg ob der Tauber (travel on a Thursday); Rothenburg ob der Tauber to Salzburg (travel on a Friday); and Salzburg to Munich (travel on a Sunday). Would point-to-point tickets purchased on the day of travel at the starting city be easiest? Are there any railpasses that would make much difference in the overall cost of the tickets? Do we really need reservations? These are the final details to plan for our trip next month.
German Rail does have an offer called "Gruppe&Spar for travel by six or more people in a group. For express trains (ICE/IC/IR/EC), the discount is 30% to 70% depending on how far in advance you purchase. For travel on regional trains, it's 50%. You can get the regional discount (and maybe the 30% express one) at the station an hour before travel time. But to get the higher express train discounts you have to make arrangements with the Bahn in advance (14 day I believe for 70%). I really don't know all of the details, as I usually am traveling alone or with one other person. However, there are other reduced rates, particularly the Länder-Tickets. Länder-Tickets are valid for unlimited travel all day (after 9 AM workdays) in 2nd class on regional trains within a single German state (Land, pl. Länder), up to five persons for around €30. For two of your legs, Rothenburg-Salzburg and Salzburg-Munich, the Bayern-Ticket for €29 should be of particular interest to you. If you buy two ticket for €29 it will cover five of you. Buy another for €29; the whole group is covered for €58. That's just €7,25 per person. If you leave Rothenburg at 9:06, you can travel by regional trains through Munich and get to Salzburg at 14:42, only 33 min later than by express train, for which the full fare is €66 per person. The trains from Freilassing, Germany, to Salzburg are run by German Rail, so Salzburg is considered a "border station" and travel on that line is included on the Ticket. For the trip back to Munich, buy another set of Bayern-Tickets. Regional trains only take ½ hour longer than express. With Länder-Tickets, there is no need for advance purchase. They are always on sale for that price at the Bahnhof on the day of travel. The price never goes up. There is no need for, or possibility of, reservations on Regional trains.
Cologne to Rothenburg is a little more problematic. By regional train, that trip will take 6½-7½ hrs, but you can do it for €120 with Quer durchs Land Tickets (2 @ €60 for 4 P), which are like Länder-Tickets except for all of Germany, not just a single state. On the other hand, a connection including ICEs would take 4-5 hours and cost €98 pP. Using somewhat slower IC/ECs would take 6-6½ hrs and cost €73 each. There are some really good discounts for advance purchase on the Bahn website for connections using at least one express train, starting at €29 pP. However you can only purchase these tickets for up to five people at a time on the Bahn site. There are only so many tickets at the starting price and when they are sold out, the price goes up, and the tickets are train specific. So, you buy, say, a ticket for five people at one price, but that uses up all the places at the lower price. When you go to buy for the next three, the price is higher. Now you have five people on that train which commits you to it, no matter what the price. Right now there are discount tickets for 5 people for the 8:53 ICE out of Koeln on Oct 13 for €209 (€49 for the first person, €40 each for 4 "Mitfahrer"). Even if the price goes up €10 per person it would still only be €159 for the last three. In a worst case scenario, you would have to pay full fare, €98 pP, for the last three.