와주신 can be directly translated into "for coming". You see usually kpop stars they will tweet things like "와주신 팬분들 고마워요~" which also means "Thank you to the fans who came". 와 - 오다 - Come 주다 - Give Therefore, 와주 - The action of "giving" to come, also coming for the person who said the sentence. 시 is added for honorific purposes, also for respect. Koreans use 시 (세요) often because of the culture that emphasizes respect in the country (especially towards strangers or persons of higher authority or experience/age) Therefore you will have 와주시. So when you tell someone "와주세요", you are simply asking someone politely to "Please come". So where does the extra -ㄴ comes from? Why is it 와주신 and not 와주시? This might be a little harder to grasp if you have not study Korean grammar to a certain level yet. In Korean, we have three different grammar forms for three different time phases, namely the past, present and future. For the past, we add -ㄴ. For the present, we add -는. And for the future, -을. So if I am to say: The person who came - 와주신 사람 The person who comes - 와주시는 사람 The person who is coming - 와주실 사람 Similarly, if you wish to omit 시 because he/she is a close friend, you will have: The friend who came - 와준 친구 The friend who comes - 와주는 친구 The friend who is coming - 와줄 친구 So in this case, the kpop star is thanking the fans who have CAME for his event. Therefore he used 와주신. Hopefully this is of help to you!

It would be easier for me to tell if you gave the context some sentences that you saw it in. What it can mean is "when you come".