Hi

I believe this to be a bug/glitch that is being fixed at the moment, if you are wanting to group up with your friends you could try going to menu - social - group - invite player and then enter in their PSN ID and hopefully this will then allow you to see them.
Another method you could try would be to exit the game and try restarting it, hopefully this will work for you but I do know that a patch is coming out which will hopefully fix this soon.

Hope this helps!