Why don't you prepare dinner _________ we may receive friends tonight?
So that or since?
It depends on the larger context. We use since to give the reason for an action. If we use it in your sentence, it would mean that we know there is a possibility that we will receive friends. By using so that, you are suggesting that if dinner were prepared, you would be able to receive (or invite) friends.
Either way, it's a pretty unlikely sentence.
Using so that, how could I rephrase this sentence and make it sound more natural?