There a couple of things to do to tenderize a grass fed steak. Unfortunately, none of them are as simple as tossing on some salt and throwing it on the grill like corn fed beef. Basically we have manual, chemical, and one quirky method.
The manual method is the easiest and by far the quickest. You can pound tenderize, but that's going to entirely destroy the joy of a thick cut steak. Instead, invest in a needle tenderizer. It looks like this and shouldn't run you more than about twenty to thirty dollars. This method is probably the best as it is going to leave the steaks flavor totally alone but does have the downside of introducing any bacteria on the outside into the interior, so you really need to make sure that your meat got sourced from someplace that handles it properly.
If you don't want to invest in a tenderizer, the next step is chemical tenderizing. In other words, marinate that bad boy. This method gives you a great opportunity to add some flavors to your meat, and I have found that herbs with some vinegar will work great with grass-fed. I would recommend trying rosemary or thyme. You will need at least 2 hours of time to properly marinate.
The last method I have I'm still working the kinks out of. It involves quick freezing the beef to allow the beef's water to tenderize the steak from the inside out. The problem with this method is you need to freeze the meat quicker than you can manage with a home freezer, but if you have access to a blast chiller it can work great. You will have to freeze and thaw and refreeze to get the crystals big enough to do the job. I only include this for completeness, as it isn't perfect and you use it at your own risk.