Lulia is right but it can be used in other ways not just cooking and friends. For example; The teacher added/threw a little chemistry into the mix. The musicians were performing a rock concert but to the crowds surprise they also threw a little classical into the mix. Basically when ever you add something to something else.

It means that you are adding another idea to an ongoing discussion. The mix is the ideas already under discussion - you add something you bring your own idea and throw it in and this adds to the 'mix'.

If it's about cooking, it simply means to add an ingredient to the mixture; otherwise it means, in a very informal way, inviting someone to join in with your friends(the mix precisely). Hope u get it! :)