Great modern city all amenities available. Elelctrical service is based on Briish system i.e. 220 volts AC (USA is 110 volts AC) so you may need convertors.
You can hire Cellphones (Mobiles) at airport or check if yours is compatabile here. The whole country is covered.
Great food from sushi, indian, mexican, british and even hash browns for breakfast, you will find Mac Donalds, KFC on every street corner.
Most water is drinkable and Bottled water available everwhere.
Exchange rate currently is 6.2 Rand to 1 us $. (If a burger costs you $4.00 in the usa you will get a good burger here for R25.00)
Currently (1st March) Hot with a lot of rain, April/May very comfortable climate going into winter June / july. Public transport not the best but I assume you have been invited so they will look after you tranport wise.
I have lived here for many years and love it.
Make sure your friends take you to a Big 5 Game reserve, (elephant lion etc)the nearest is Pilansberg 2 hours from johannesburg. Have a great stay. c