All istanbul traveler groups with more than 4 persons needs to organize a private transfer to the Istanbul Airports. You can book a safe Istanbul Airport transfer at one of your prefered and trusted transfer company.
Be careful to use uber cars; because uber is not allowed and forbidden in Turkey, finally if you will stoped by the traffic police you will lost a lot of time and money; don’t waste your time and money for 10 euros! Also the guests will punished by using uber cars or other not allowed transfers.
The another way is to use public transportations, but this takes to long and its very hard for unexperience people.
Even be careful to use the public yellow cabs, because they will not happy about to drive to the new Istanbul airport because, they will risk an empty drive back for 1 hours – in this case they will lost a lot of time and money. May the risk or luck for unwanted sightseeing will increase.