If you ask me (and you indirectly did :-P ), I understand the offer as follows:
If you travel FROM Germany TO Austria, Belgium, Denmark, Italy, Luxembourg, Netherlands and Switzerland you can use the "Saving price 50 Europe". That will allow you to reduce the price FOR THE SECOND AND FIFTH PERSON accompanying you.
On GERMAN railways you need to have the BahnCard. X-O
If you order via Internet they of course show you the savings button for all those people having the BahnCard.
The German Railways NEVER give you a 50% discount for about nothing (own experience). *sad*
That are my impressions.
If you want to be absolutely sure you should go and ask someone at your local travel agency.
I checked your route: You mentioned Frankfurt, but you are traveling via Stuttgart Airport, arent you? Be aware that you choose the right airport station then (they let you choose from STR StuttgartAirport and Flughafen/Airport Stuttgart, Leinfelden-Echterdingen - don