One point missed by this article (but mentioned by another commenter) is that it's not enough to just want more friends. It's not even enough to meet another person who also wants friends. What keeps friends together is perceived mutual exchange of value - it sounds callous, but I challenge anyone to describe a friendship that lasted where either party believed they derived no value.

Our lizard brains aggressively pre-filter potential friends by estimating expected value add. One property that contributes to perceived value is scarcity, and this leads to a sort of counterintuitive result: it may be easier to make friends if you don't really want or need them (or at least you appear that way). If you are self-reliant, happy, and occupied most of the time, your time and attention are scarce (from the perspective of others). This signals value and makes others want to be your friend. Obviously, this doesn't mean you should be constantly unavailable. But it does mean that being constantly available to hang out, talk, do favors, etc. will probably make you less desirable as a friend.