When we went from 5 to 6, we called and they made the change with no problem. When we found out one more person was coming, we called again to change to 7. They couldn't do it. The cars seat 6 so adding a 7th person really, really limited where they could fit us. They said we'd probably be split 4 and 3. We chose a different meal to have the others join us at, and went back to 5. Even so, with our ADR and back to 5 people, we waited the longest there than for any other ADR to get a table.
That place is crazy busy. I wouldn't just show up with 7 since it might be impossible to seat you. Definitely call. You might have to just adjust your ADR time. Or have the extra 2 see if they can get an ADR for themselves at Sci-Fi for the same time.