Go to meetups, conferences, etc and just mingle and socialize. Just talk to people and get to know them. There's a monthly lunch and learn near me for infosec and the number of people you meet and network with in a semi formal setting is great. People show up who aren't even part of the industry but want to learn. Has definitely lead me to expanding my network of contacts.
Metro areas will have meetups for entrepreneur/startup folks, eg Seattle has 200+! Find some you like and go, hand out cards. Try a few and refine, attend each a few times, to get a good evaluation of the group. Friend up on LI/Twitter,etc, build network, find possible founders, lather, rinse, repeat
There are a few ways.
1. Join a large company that recruits extraordinary talent, but has many startup founder alumni (e.g. Google, Facebook, Uber).
2. Join a startup, work for a few years. After an exit, start a company with your colleagues.
3. Go to school and meet your cofounders there.
One of the things you could try is planning a lunch or dinner or trek or some other activity that is enjoyable and sort-of informal-ish. Invite 1-2 of your founder friends and ask them to bring 1-2 of their friends along. Make sure the group doesn't become so large that members subdivide into subgroups. This way not only you, but your other founder friends can expand their network. And, if on the off chance that someone doesn't get along well with someone else, they still have someone in the group to enjoy the fun activity with.