The user always enters 5 friends, right? So use for (int counter = 0; counter < 5; counter++).
Then you should have a 5-element array of strings for the friend names (you already do have this, but it's named incorrectly: 'num_friends' shouldn't be a string, and it should just be called 'friends' or 'friend_names' so that its name represents what it actually is).
The random seed you're talking about comes after the user has entered all five names. You can let the user enter a seed, then seed the random number generator with that, if you'd like. See http://www.cplusplus.com/reference/cstdlib/rand/ for some help with that.