Why stand there? Come here! This is okay to speak and is used often in this form. Why are you standing there? Come here! Standing in this context is a gerund naming the activity the person is doing.

Almost! Why are you standing there? Come here. In English, you almost always need the subject in the sentence (in this case, "you"). It usually is wrong without them. But not always, since the command or suggestion "come here" is fine.

The right sentence is the second. But I think with a little remark. Why are you standing there? This time is Present Continuous. Action is happening in this moment

Sawasdee Krab, Matt. When you look at someone and talk to him/her at the moment, then you need to use present continuous tense. " Why are you standing there? " is the correct sentence. And, if you invite him/her to your house, you might want to say " Please come in ", or just simply say "Please come over here ". Those sentences are more polite. Hope it helps, Tere