It took me about a year of trying different things in my area, but I've got a couple groups going. I volunteer with the local NICA team, so I do rides pretty regularly with them. Bunch of kids, and a decent number of coaches are pretty regular. Just got back from a ride with them. These rides tend to be shorter, so they're good for just getting my frequency up.
I've found some other guys who like to ride fairly regularly, too, and we've been working on getting something regular going. It's been tough with the holidays, bad weather, and the gov't shutdown preventing some necessary storm cleanup, but we're getting out tomorrow and it looks like our group is going to have 10 people. It's only ever been 4 at the most before, so I'm hoping the group dynamic is a good one.
I also ride with my wife. She and I are going to be riding on Monday, it looks like. She has plans tomorrow so can't make it to the group ride. But she rides with the group when she can.
I have tried some group rides off the local meetup, and they DEFINITELY didn't fit me well, so I was a single time attendee on those. Sometimes you can find groups on Facebook and there's one I was put in touch with from a fellow mtbr user awhile back that's a good group I'll ride with sometimes. I'm on a couple other local Facebook mtb groups, but they don't usually put on "group" rides per se...it's mostly just people talking bikes and occasionally asking people to ride with them. They do get takers, so that's definitely an option if you're hankerin for a group and your schedule is weird. I haven't tried any of the local shop rides, but that's a scheduling issue more than anything.