Anyone and everyone. At the most simplistic level, the more people that you are connected to, the more you stand to learn and the greater your odds of finding out about all sorts of opportunities.
Of course, you don’t want to be spending your precious time chit-chatting to people for the sake of it. But, neither do you want to be so single-minded and rigid with your networking that you miss out on potentially valuable contacts. Even people who have nothing professionally in common with you have their own networks, and might know someone who does. You may have a few seemingly fruitless conversations from time to time, but you will also probably find opportunities you never even knew existed, often through and with the least likely people.
So spread your net wide. Don’t limit your network to current colleagues: past employers, colleagues of colleagues, friends, family and just about anyone you meet can form your network. If you’re a graduate, school leaver or new jobseeker, your teachers, lecturers or professors can prove invaluable – they know your strengths as well as anyone and will often have expert contacts in your chosen field of work.
And keep growing your network: ask anyone you’ve met if they can think of anyone else it would be useful for you to talk to (could they even introduce you?): any networking that results in another lead has been a success.