1. Find a group of knitters at a variety of skill levels.

2. Meet somewhere where you won’t annoy other people.

3. Let people know they can join.

4. Make a Google Group to keep in contact.

5. Don’t let the haters cramp your style.

Because who knows yarn better than baby cats? No one, that's who. Got tips? Contribute below.
That way, when you totally fuck up your piece and drop ten stitches, someone’s there to fix your problems (if you don’t know how to knit but for some reason want to start a knitting group, start here).
Cafes seem like a cute idea, until you’re all there for four hours with your single cups of coffee. That’s not going to make the wait staff happy. Some craft stores have knitting groups, or would let you start one, but then you can’t make your favorite cups of tea (or drink wine. In moderate amounts. Because too much wine equals too many crossed stitches). Someone’s home is a good idea, if they have enough seating to go around and don’t mind being invaded and don’t have pets that will cause too much chaos.
This is assuming you don’t set up a secret password and initiation rites. We’re assuming that you know how to advertise to your friends via Facebook/Twitter/Etc. You can also ask your local yarn and craft store if you can post a flyer with them, or anyone you happen to see knitting in public.
This will help you with all those pesky logistical issues, like when to meet, how long to meet, and where to meet. It will also let you spam your fellow knitter’s emails with questions about gauge and cabling. Also probably pictures of cats. Learn how to start one here.
But really. Ignore what your so-called friends say, knitting is not just for grandmothers anymore. Even Ryan Gosling does it.