In the USA "What's up for dinner?" means: "What are we doing for dinner?" -so- It has multiple meanings. It means "What are we eating?" and "Where are we eating?" and "Who are we eating with?" One may also hear: "What's up WITH dinner?"

I have never heard the phrase "Whats up for dinner?". It is common to say "Whats for dinner?" when asking what you will be eating for dinner. However, you might have heard someone say "Whats up?" which is the same as asking "How are you?" or more often "Whats wrong?". Hope this helps!