On the Beijing Yoshinoya website they have a number 400 819 7197 for delivery. I assume the one close to your hotel is 北京市海淀区中关村大街西区鼎好电子商城5层? Their direct number is 010 8269 6621, but better call the 400 number as it is specifically for delivery.
But if you do not speak Chinese I am not sure how you can actually order, since they may not speak English. Anyway the food in Yoshinoya is not that amazing.

BTW their website is pretty hopeless...