PS4 Can't play with friends ... how to fix
I like many other people on the PS4 are having problems playing with their friends.
The problem is down to the size of either your friends list or the size of your friend's friends list (I've had this problem on other games too). It seems defiance only recognizes 100 of your friends (which was the max amount of friends you could have on the PS3). Even if your friends list is less than 100 you can still have problems playing with people who have over 100 friends - I know this because my brother-in-law has less than 100 friends but he couldn't see me in game.
The hard solution to this problem is to whittle down your friends list getting rid of people you no longer play with, but that won't work if the problem is with your friend.
The easy solution to the problem is to remove the friend you want to play with from your friends list and send them a friends request - the reason being is that defiance only sees the 100 most recent friends on your friends list. This should solve the problem (it did for me).