I agree with Shazerac! Doing something you enjoy with like-minded people is going to widen your circle of friends/good acquaintances and even if you do not meet your potential boyfriend at the actual meetups, chances are you meet someone who will introduce you to him in future

As for "do you think it happens naturally or would you rather have to work for it?", balance is the key. Being too obsessed about finding someone is going to wear you out and create a heavy energy around you. It may also lead you to rushing into things with someone you don't know that well or who is not actually meeting your needs and standards.
Hoping that things just happen naturally may cause missed opportunities...

So what to do?? Do join these meetups/activity clubs/volunteering causes, do reach out and talk to people you feel attracted to but also be patient and calm, know deep down that you WILL indeed meet this special person and give yourself (and them) time to open up. This openness and calm confidence will attract someone very interesting your way.

Good luck!