Location, location, location...one of our knitting groups folded after a few month, the other is still going strong. I think there were several factors involved, but location was the biggest.
One was held in a well-lit cafe (there are extra lights that get turned on when the knitters are there) with two comfy couches and plenty of room to pull up additional chairs (different kinds of seating is a plus). The cafe has a good menu with lots of choices (food and beverages, sandwiches, salads, coffee, wine, beer, etc). Music, but not so loud that it makes conversation difficult.
The other was held in a great Mission coffee house with great music, but the lighting was poor (Bill used to carry spare light bulbs in his knitting bag), the seating was uncomfortable and if it was crowded, we couldn't all sit together. The food and coffee were fine, but limited to coffee house fare. I like this place a lot and I'll go there to knit by myself sometimes, but it doesn't work for a group.
Another factor was time. There was a group on Monday night and one on Tuesday night (in fact, I think for a while anyway, you could find a different group to knit with every night of the week). Monday Night Knit vs. Men Who Knit and many of us went to both groups for a while. But when I needed to pare down to one night a week, I chose Monday. I would really like to see a men's night start up again, but as a monthly event.
On location, again, the staff also makes a big difference. Monday Night Knit was started by the cafe owner who is also a knitter. He understands the need for good lighting. He reserves an area for the knitters so we don't have to worry about beating the crowds to the cafe and staking out our territory. And he has great people on his staff. He even knits with us, but not often enough.
I think men's knitting groups are a great idea and I don't want to discourage them, but one of the reasons I love my knitting group is the people who are there. It's a terrific mix of great people. If you limit your membership, you limit your group. You limit your potential for success, you limit your potential for inspiration, great conversation, growth, etc. Like I said, I would love to see a monthly men's knitting night in San Francisco, but for a weekly group, I think we've got it pretty good.