It took me years to find a really good writing group, and now it seems as if it takes years to set up a writing group meeting. But I stick with it. I love my group, and it has helped me improve my writing tremendously. I've learned so much! (Probably no coincidence that a Scholastic editor at an SCBWI manuscript critique asked if if I was in a critique group--I wasn't at the time, and it showed.)
For anybody looking to join or start a group: find people who aren't afraid to tell you what needs improvement. Find people who will listen courteously to what you say (as you will listen courteously to them. Find people who will support you in your writing, instead of tearing you down, and rejoice at your good news. Above all, find people you like to be with!