Visiting Hamburg would give you a chance to see a little different part of Germany as, for centuries

I am going to try and recommend things that are easy to plan and execute by train. There are certainly a ton of great places to spend 2 days along either route, but not all of them are going to be a comfortable goal for 15 people traveling together.
You can travel by train from Berlin to Boppard in about 6 hours with 1 change in Frankfurt. There are other options for a change, like Cologne, but Frankfurt is the fastest. You can also travel from Munich to Berlin direct, with no changes, in about 6 hours.
Since the Munich to Berlin portion is a little easier, without travel near train near italy a connection required, it might make sense to take advantage of that and not stop along the way. Breaking up the trip to Boppard, which requires a train connection anyway would seem to "interrupt" your trip transportation less. However, the rail stops along the various routes between travel near train near italy Berlin and Boppard aren't the most interesting places for a group of 15 to stay for 2 days - depending on what you want to see, of course.
Visiting Hamburg would give you a chance to see a little different travel near train near italy part of Germany as, for centuries, it was part of the Hanseatic League involved in the North Sea trade. It really is a beautiful city worth a couple of days.
Bamberg is on the train line between Munich Hamburg; Bamberg has a great center, and is also wonderful for beer (unfiltered Kellerbier; smoked Rauchbier); you could side-trip to Nuremburg or Wurzburg from there easily too (each are under an hour by train),