If you're cooking it by a moist-heat method (like simmering meatballs in a sauce, for example, where there's liquid added) then you may just need a longer, slower cooking time. Grass-fed beef likes to cook slower and longer to stay tender and moist.
If you're cooking by dry-heat methods (like baking or broiling, for example, or pan-browning) you might want to add a little extra fat to the meat. Choose good natural fats like butter, palm oil, or olive oil. (One of our favorite meatloaf strategies actually is to add mashed avocado to the mixture, which adds lots of good fats.) Grass-fed beef is lean, and the extra fat will help keep it tender and moist. Also, try covering it for part or all of the cooking time, to keep the moisture in, and cooking more slowly at a lower temperature, which means cooking for a bit longer.