What would a taxi cost from the airport to the Yiyang City Center Apartments, 40 Dongzhong Street for 5 people?
About RMB 80 on the meter + RMB 10 expressway toll added on at the end. Note that standard Beijing taxis cannot take 5 people, so you'll either need two standard taxis, or ask for a larger van-type taxi that can fit the entire family plus luggage. This will be somewhat more expensive than a standard taxi, but shouldn't be more than splitting up and taking two standard taxis.
Correct, yo need two taxis. A minivan would be about 350RMB.