Originally Posted by thijsseh

Having 4 paid J tickets and assuming they are not ‘special’ fare (in which case you are out of luck altogether), you can book 4 cars in each of MIA, JNB, CPT and MIA again. You do this via manage my booking on the Emirates website. Often you can get a minivan or other large car to transport all of you, but you willl have to arrange that with the service provider. Generally, you will get an email, a text message or a phone call between 24 and 48 hours before the flight giving you the opportunity to agree on what suits best. Of the stations you mention, I only know CPT (very well). The service is provided by Avis point to point and is very professional. They will contact you via email and give you a phone number so you can talk to them.
In a case like yours, and not knowing how the service is arranged in the stations where you go, I personally would book 2 cars (which will easily take the 4 of you plus the luggage) and when you get contacted you can ask for a van or something like that and cancel the other car.
As far as DXB is concerned: is the 13 hour connection the shortest possible? In that case you qualify for a car, hotel and meals at the expense of the airline. If you qualify, you can book this via MMB and you will be given vouchers when you check in in MIA (and / or CPT). If you don’t qualify for this service, you don’t get a car for a 13 hour layover.