Just out of curiosity, how would you control someone well in open guard? or how did the people control them?
In an open guard, there are lots of variations besides just laying on your back with your feet on the ground. Here are some examples.
1) The ever popular butterfly guard. This guard is achieved by hooking your feet in the inside of your opponent's thighs. This lets you control his legs and his base with your feet, while you control his head and arms with your own head and arms. Because your feet are hooked in, and because your feet are now directly affecting his base, you can use your feet to throw him off balance while you sweep with the assistance of your arms. Also, your feet are basically available for all sorts of transitions - you can transition from butterfly guard to rubber or spider guard, you can avoid all sorts of strikes since you have control of his base and his arms, and you can attack with armbars and triangles and omoplatas and all that other good stuff once you figure out how to control your opponent to your advantage.
2) The spider guard. If you want some space, you can use the spider guard (feet on his hips, arms, legs) to keep your opponent from advancing on you. This creates opportunities because he'll have to find a way inside, and he'll usually wind up exposing an arm for a kimura or armbar or triangle - you'll be able to see it coming. If you grab at your opponent's sleeves and his head with your hands, and keep your feet on him and your base always in front of him, he isn't going anywhere. You can create all sorts of opportunities for attacks too - you can push with your feet so he resists and pushes forward on you - keep a hold of one of his sleeves and suddenly release your feet from him so he comes diving into you - quick armbar right there. You can use similar movements to keep him constantly off balance and always guessing at your next move.
3) X Guard - this is basically a guard where your opponent is standing and you're trying to wrap up his leg. See Frank Mir vs. Brock Lesnar - he only had it for a second, but you see what it led to - a powerful sweep into a submission. You can wrap up his leg and pull him back down into almost any type of guard without suffering much damage. So even if someone is able to "escape" your open guard by standing up, he's really still in your open guard if you can wrap up his legs.
4) Rubber Guard - Do I need to say more? Being put into a good rubber guard just sucks (one leg high up on your back to keep your head down). It's like the guy you're rolling with has 3 arms to attack you with.
It's really the constant transitions from those 4 guards that are dangerous in regards to the open guard. If someone's just sitting there with one type of open guard, or even worse, the basic lay-on-your-back-with-feet-on-floor guard, it's super easy to pass. But when faced with someone using all 4 effectively, it's really, really hard to get away (until you inevitably get swept or submitted). Besides, nearly every submission from guard requires you to open the guard first - so when you're sitting in an open guard, you can pretty much be attacked at any time.
Just to be super clear - here's an example of how you would control someone with open guard.
Open my closed guard, try for a kimura. He grabs his own forearm to defend, but now he's close to me, so I hug his back to keep him from sitting up, shift my hips back just a smidge, maneuver hooks in and have thus applied the butterfly guard. I let go of his back, he tries to pass to the right. I grab his sleeves and constantly attack his base so he can't get enough momentum to explode away. I threaten a sweep to the left by lifting both legs but exploding with my right while simultaneously using my arms to turn his body towards that side, but he somehow escapes from both hooks and stands up (hey, things happen while grappling). I immediately wrap his leg at the knee with both my legs, and send him back to one knee and immediately grab at his gi. I pull him down using my body weight for assistance while shrimping my legs out to put him back in my open guard. He tries to stand back up, but I grab his head and a sleeve and pull him back down. He tries to push forward, so I stick my feet on his hips and push my hips back, creating a little bit of space and therefore using the spider guard. Now my feet are blocking his forward movement. He tries to push forward, I let go of my left foot and grab his right arm. Now he's off balance on his right side and both my feet are basically free, with my left foot already in transit. I just shift my hips, swing my feet over his head and go for an armbar. If I miss and his arm slips out, I switch out my left leg, keep my right leg high on his back and keep him down. There's the rubber guard. From here, whenever I feel like it, I can let him up a bit, slip in a shin and try for a gogoplata. Or, wait for him to plant an arm and go for a kimura. Or, shifting my leg from on his back to over his arm and trying for an omoplata.
I'm not good enough to do all that, but I know enough to know that similar things have been done to me. Of course the best strategy is to incorporate the closed guard too, so that you'll have a truly ultimate guard that no one will ever ever want to be caught in, but that's a different story. Well, I hope this super long treatise on the art of BJJ from a novice grappler has helped (I'm more of a stand-up fighter, but I'm good at figuring out strategies and how things work). Good luck in all your martial arts related endeavors!
