For local, keep posting on craigslist.
Following are some organizations that offer the opportunity for musicians to get together in more-or-less structured environments in the New York City area. They do cost some $.
For Jazz
- New York Jazz Academy (they have classes on Long Island)
- New York Jazz Workshop
- SOJ Jazz Center at the Collective
- Lucy Moses School (Search under Music->Jazz)
All the above offer workshops and ensembles - the first three especially have different ensembles you can try to get into.
For Rock
- The Jam NYC (Jam sessions and networking - chance to form your own band)
- The Studio (Structured workshops and jam sessions, and probably the chance to network as well)