Preen is a pre emergent control so it will not keep already growing anything from growing. If this grass is one of the invasives then the only method of control is to dig it out, every bit of root, tiller, rhizome that you can.

A picture would be helpful. If it is a millet, I doubt that Preen will prevent it from germinating and growing, and as Kimmsr says, anything already emerged will not be touched. Grass killers such as Grass-B-Gone will take care of emerged plants as long as what you are protecting isn't also susceptible to it, and beans are one of the plants it shouldn't hurt. Your best long term solution is to prevent seed production. Even if you don't get them all pulled, don't allow any of them to go to seed--pull the seed heads out as soon as they show up.

Vinegar is also a non-selective chemical (beware getting it on any of your desired plants, and it can affect the roots also), but is so common it seems comfortable whereas the pesticides aren't so familiar.

In a garden, it is often impractical to use herbicides as you have a large variety of vegetables and very few pesticide choices that won't hurt at least some of them. Tillage (hoeing, cultivating) when they first come up is usually the most effective, as are mulches.