Ok it seems like 1-2% is best then, thank you
And just to clarify, as this is a carrier oil all I need to do is add it in and mix it thoroughly, doesn't need to be heated or combined with anything else right?
Zany is right. Argan oil is better as a leave-in hair treatment. It washes out with shampoo and doesn't deposit on the hair. Shampoo is designed to remove oil from hair, not to leave it there.
However, if you want to add it anyway for "label appeal", read on.
Depending on the surfactants you use to make your shampoo, you might need to add a solubilizer, or you might already be using a surfactant that also acts as a solubilizer. What I would do, is research your other ingredients to see if something already in your formula will solubilize added oil. Keep your total oil percentage really low, as low as you can, or it will decrease your lather. You will probably be able to mix it in with the other ingredients (after blending with the solubilizer first) but you may need to warm it up slightly if it doesn't blend well at room temperature. If it won't blend in at all, or separates after a few days, you need either more solubilizer, or a different kind, or less oil.
You probably already know this, but I'll mention it just in case: If you do end up adding a solubilizer, make sure you test the pH of the finished product with a good pH meter to make sure it's still below 5 or so. The solubilizer I'd use in shampoo is Caprylyl Capryl Glucoside, but it has a very high pH, which even in small amounts will cause the pH of your shampoo to be higher than is healthy for hair. This is the one I use in my laundry detergent (true detergent, not soap) to add fragrance, and I use it partly because it gets the pH up high enough than I don't need a preservative. The pH of my finished detergent is usually between 10.5 and 11, which is close to the pH of soap, which also doesn't require preservatives for the same reason.
If you are making a lye-based soap, I wouldn't use Argan oil at all. It will saponify and the soap version of Argan oil has different properties than the oil itself. If you have done your research and you are aware of the issues regarding soap as shampoo, and you want to try it regardless, I can't help you because I can't use soap as shampoo. Like most people, my hair type would not tolerate the alkalinity of soap, even with an acid rinse. Zany has posted some recipes here however if you want to go that route.
If you are using surfactants to make your shampoo, the area we usually use for discussing that type of product is the "Bath and Body Forum". At the bottom of your first post in this thread, on the bottom left is a link that says "report". Click that and ask a mod to move this thread to the Bath and Body area.