Welcome. First:
How many is "we"? Do you have flexibility to stay a few days longer if not boarded? What is the employee's date of hire (DOH)? No one an predict JNB history. Every day varies and tourism is very active on DL, as well a plenty of business travelers. Ask you employee to monitor flight for a few weeks keeping in mind this is winter and off season. This week has been pretty good to S4s but lots of middle seats scattered in the far back of AC and very few get to sit together.
Suggest you do search here for JNB and read first.
I would never advise our buddy pass riders to attempt ATL/JNB.
Getting back can be such a difficulty. We did have to use zeds to Dubai once and we had 25 yrs and S2 priority. I've been about 6 times and my stomach churns almost every return. Read the other posts first, then come back if you have more questions.